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The
Unicode Consortium The Unicode Consortium (legally Unicode, Inc.) is a 501(c)(3) non-profit organization incorporated and based in Mountain View, California. Its primary purpose is to maintain and publish the Unicode Standard which was developed with the intenti ...
and the
ISO/IEC JTC 1/SC 2 ISO/IEC JTC 1/SC 2 Coded character sets is a standardization subcommittee of the Joint Technical Committee ISO/IEC JTC 1 of the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC), that devel ...
/ WG 2 jointly collaborate on the list of the characters in the Universal Coded Character Set. The Universal Coded Character Set, most commonly called the
Universal Character Set The Universal Coded Character Set (UCS, Unicode) is a standard set of characters defined by the international standard ISO/IEC 10646, ''Information technology — Universal Coded Character Set (UCS)'' (plus amendments to that standard), whi ...
( UCS, official designation:
ISO ISO is the most common abbreviation for the International Organization for Standardization. ISO or Iso may also refer to: Business and finance * Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007 * Iso ...
/
IEC The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and r ...
10646), is an international standard to map
characters Character or Characters may refer to: Arts, entertainment, and media Literature * ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk * ''Characters'' (Theophrastus), a classical Greek set of character sketches attributed to The ...
, discrete symbols used in
natural language In neuropsychology, linguistics, and philosophy of language, a natural language or ordinary language is any language that has evolved naturally in humans through use and repetition without conscious planning or premeditation. Natural languages ...
,
mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
,
music Music is generally defined as the art of arranging sound to create some combination of form, harmony, melody, rhythm or otherwise expressive content. Exact definitions of music vary considerably around the world, though it is an aspect ...
, and other domains, to unique
machine-readable data Machine-readable data, or computer-readable data, is data in a format that can be processed by a computer. Machine-readable data must be structured data. Attempts to create machine-readable data occurred as early as the 1960s. At the same time tha ...
values. By creating this mapping, the UCS enables computer software vendors to
interoperate Interoperability is a characteristic of a product or system to work with other products or systems. While the term was initially defined for information technology or systems engineering services to allow for information exchange, a broader defi ...
, and transmit— interchangeUCS-encoded text
strings String or strings may refer to: *String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects Arts, entertainment, and media Films * ''Strings'' (1991 film), a Canadian anim ...
from one to another. Because it is a ''universal'' map, it can be used to represent multiple languages at the same time. This avoids the confusion of using multiple legacy
character encodings Character encoding is the process of assigning numbers to graphical characters, especially the written characters of human language, allowing them to be stored, transmitted, and transformed using digital computers. The numerical values that ...
, which can result in the same sequence of codes having multiple interpretations depending on the character encoding in use, resulting in ''
mojibake Mojibake ( ja, 文字化け; , "character transformation") is the garbled text that is the result of text being decoded using an unintended character encoding. The result is a systematic replacement of symbols with completely unrelated ones, ofte ...
'' if the wrong one is chosen. UCS has a potential capacity of over 1 million characters. Each UCS character is abstractly represented by a
code point In character encoding terminology, a code point, codepoint or code position is a numerical value that maps to a specific character. Code points usually represent a single grapheme—usually a letter, digit, punctuation mark, or whitespace—but ...
, an integer between 0 and 1,114,111 (1,114,112 = 220 + 216 ''or'' 17 × 216 =
code points In character encoding terminology, a code point, codepoint or code position is a numerical value that maps to a specific character. Code points usually represent a single grapheme—usually a letter, digit, punctuation mark, or whitespace—but ...
), used to represent each character within the internal logic of
text processing In computing, the term text processing refers to the theory and practice of automating the creation or manipulation of electronic text. ''Text'' usually refers to all the alphanumeric characters specified on the keyboard of the person engaging t ...
software. As of
Unicode Unicode, formally The Unicode Standard,The formal version reference is is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expre ...
15.0, released in September 2022, 293,168 (26%) of these code points are allocated, 149,251 (13%) have been assigned characters, 137,468 (12.3%) are reserved for private use, 2,048 are used to enable the mechanism of
surrogates ''Surrogates'' is a 2009 American science fiction action film based on the 2005–2006 comic book series '' The Surrogates''. Directed by Jonathan Mostow, it stars Bruce Willis as Tom Greer, an FBI agent who ventures out into the real world to ...
, and 66 are designated as , leaving the remaining 820,944 (74%) unallocated. The number of encoded characters is made up as follows: * 149,014 graphical characters (some of which do not have a visible
glyph A glyph () is any kind of purposeful mark. In typography, a glyph is "the specific shape, design, or representation of a character". It is a particular graphical representation, in a particular typeface, of an element of written language. A g ...
, but are still counted as graphical) * 237 special purpose characters for
control Control may refer to: Basic meanings Economics and business * Control (management), an element of management * Control, an element of management accounting * Comptroller (or controller), a senior financial officer in an organization * Controllin ...
and formatting.
ISO ISO is the most common abbreviation for the International Organization for Standardization. ISO or Iso may also refer to: Business and finance * Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007 * Iso ...
maintains the basic mapping of characters from
character name Character or Characters may refer to: Arts, entertainment, and media Literature * Character (novel), ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk * Characters (Theophrastus), ''Characters'' (Theophrastus), a classical Greek ...
to code point. Often, the terms ''character'' and ''code point'' will be used interchangeably. However, when a distinction is made, a ''code point'' refers to the
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign (−1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...
of the character: what one might think of as its address. Meanwhile, a ''character'' in ISO/IEC 10646 includes the combination of the code point and its name, Unicode adds many other useful
properties Property is the ownership of land, resources, improvements or other tangible objects, or intellectual property. Property may also refer to: Mathematics * Property (mathematics) Philosophy and science * Property (philosophy), in philosophy and ...
to the character set, such as
block Block or blocked may refer to: Arts, entertainment and media Broadcasting * Block programming, the result of a programming strategy in broadcasting * W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...
, category,
script Script may refer to: Writing systems * Script, a distinctive writing system, based on a repertoire of specific elements or symbols, or that repertoire * Script (styles of handwriting) ** Script typeface, a typeface with characteristics of handw ...
, and directionality. In addition to the UCS, the supplementary
Unicode Standard Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, whic ...
, (not a joint project with ISO, but rather a publication of the Unicode Consortium,) provides other implementation details such as: # mappings between UCS and other character sets # different collations of characters and character strings for different languages # an algorithm for laying out bidirectional text ("the
BiDi A bidirectional text contains two text directionalities, right-to-left (RTL) and left-to-right (LTR). It generally involves text containing different types of alphabets, but may also refer to boustrophedon, which is changing text direction in eac ...
algorithm"), where text on the same line may shift between
left-to-right A writing system is a method of visually representing verbal communication, based on a script and a set of rules regulating its use. While both writing and speech are useful in conveying messages, writing differs in also being a reliable form ...
("LTR") and
right-to-left In a script (commonly shortened to right to left or abbreviated RTL, RL-TB or R2L), writing starts from the right of the page and continues to the left, proceeding from top to bottom for new lines. Arabic, Hebrew, Persian, Pashto, Urdu, Kashmiri ...
("RTL") # a case-folding algorithm Computer software
end users In product development, an end user (sometimes end-user) is a person who ultimately uses or is intended to ultimately use a product. The end user stands in contrast to users who support or maintain the product, such as sysops, system administrato ...
enter these characters into programs through various
input methods An input method (or input method editor, commonly abbreviated IME) is an operating system component or program that enables users to generate characters not natively available on their input devices by using sequences of characters (or mouse o ...
, for example, physical
keyboards Keyboard may refer to: Text input * Keyboard, part of a typewriter * Computer keyboard ** Keyboard layout, the software control of computer keyboards and their mapping ** Keyboard technology, computer keyboard hardware and firmware Music * Musi ...
or virtual character palettes. The UCS can be divided in various ways, such as by
plane Plane(s) most often refers to: * Aero- or airplane, a powered, fixed-wing aircraft * Plane (geometry), a flat, 2-dimensional surface Plane or planes may also refer to: Biology * Plane (tree) or ''Platanus'', wetland native plant * ''Planes' ...
, block, character category, or character property.


Character reference overview

An
HTML The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaScri ...
or
XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. T ...
numeric character reference refers to a character by its
Universal Character Set The Universal Coded Character Set (UCS, Unicode) is a standard set of characters defined by the international standard ISO/IEC 10646, ''Information technology — Universal Coded Character Set (UCS)'' (plus amendments to that standard), whi ...
/Unicode code point, and uses the format :&#''nnnn''; or :&#x''hhhh''; where ''nnnn'' is the code point in
decimal The decimal numeral system (also called the base-ten positional numeral system and denary or decanary) is the standard system for denoting integer and non-integer numbers. It is the extension to non-integer numbers of the Hindu–Arabic numeral ...
form, and ''hhhh'' is the code point in
hexadecimal In mathematics and computing, the hexadecimal (also base-16 or simply hex) numeral system is a positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system representing numbers using 10 symbols, hexa ...
form. The ''x'' must be lowercase in XML documents. The ''nnnn'' or ''hhhh'' may be any number of digits and may include leading zeros. The ''hhhh'' may mix uppercase and lowercase, though uppercase is the usual style. In contrast, a ''character entity reference'' refers to a character by the name of an ''
entity An entity is something that exists as itself, as a subject or as an object, actually or potentially, concretely or abstractly, physically or not. It need not be of material existence. In particular, abstractions and legal fictions are usually ...
'' which has the desired character as its ''replacement text''. The entity must either be predefined (built into the markup language) or explicitly declared in a
Document Type Definition A document type definition (DTD) is a set of ''markup declarations'' that define a ''document type'' for an SGML-family markup language ( GML, SGML, XML, HTML). A DTD defines the valid building blocks of an XML document. It defines the document ...
(DTD). The format is the same as for any entity reference: :&''name''; where ''name'' is the case-sensitive name of the entity. The semicolon is required.


Planes

Unicode and ISO divide the set of code points into 17 planes, each capable of containing 65536 distinct characters or 1,114,112 total. As of 2022 (Unicode 15.0) ISO and the Unicode Consortium has only allocated characters and blocks in seven of the 17 planes. The others remain empty and reserved for future use. Most characters are currently assigned to the first plane: the ''Basic Multilingual Plane''. This is to help ease the transition for legacy software since the Basic Multilingual Plane is addressable with just two
octet Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 compos ...
s. The characters outside the first plane usually have very specialized or rare use. Each plane corresponds with the value of the one or two
hexadecimal digit In mathematics and computing, the hexadecimal (also base-16 or simply hex) numeral system is a positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system representing numbers using 10 symbols, hexad ...
s (0—9, A—F) preceding the four final ones: hence U+24321 is in Plane 2, U+4321 is in Plane 0 (implicitly read U+04321), and U+10A200 would be in Plane 16 (hex 10 = decimal 16). Within one plane, the range of code points is hexadecimal 0000—FFFF, yielding a maximum of 65536 code points. Planes restrict code points to a subset of that range.


Blocks

Unicode adds a block property to UCS that further divides each plane into separate blocks. Each block is a grouping of characters by their use such as "mathematical operators" or "Hebrew script characters". When assigning characters to previously unassigned code points, the Consortium typically allocates entire blocks of similar characters: for example all the characters belonging to the same script or all similarly purposed symbols get assigned to a single block. Blocks may also maintain unassigned or reserved code points when the Consortium expects a block to require additional assignments. The first 256 code points in the UCS correspond with those of
ISO 8859-1 ISO/IEC 8859-1:1998, ''Information technology — 8-bit single-byte coded graphic character sets — Part 1: Latin alphabet No. 1'', is part of the ISO/IEC 8859 series of ASCII-based standard character encodings, first edition published in 1 ...
, the most popular 8-bit
character encoding Character encoding is the process of assigning numbers to Graphics, graphical character (computing), characters, especially the written characters of Language, human language, allowing them to be Data storage, stored, Data communication, transmi ...
in the
Western world The Western world, also known as the West, primarily refers to the various nations and state (polity), states in the regions of Europe, North America, and Oceania.
. As a result, the first 128 characters are also identical to
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
. Though Unicode refers to these as a Latin script block, these two blocks contain many characters that are commonly useful outside of the Latin script. In general, not all characters in a given block need be of the same script, and a given script can occur in several different blocks.


Categories

Unicode assigns to every UCS character a ''general category'' and subcategory. The general categories are: letter, mark, number, punctuation, symbol, or control (in other words a formatting or non-graphical character). Types include: * Modern, Historic, and Ancient Scripts. As of 2022 (Unicode 15.0), the UCS identifies 161 scripts that are, or have been, used throughout of the world. Many more are in various approval stages for future inclusion of the UCS. * International Phonetic Alphabet. The UCS devotes several blocks (over 300 characters) to characters for the
International Phonetic Alphabet The International Phonetic Alphabet (IPA) is an alphabetic system of phonetic transcription, phonetic notation based primarily on the Latin script. It was devised by the International Phonetic Association in the late 19th century as a standa ...
. * Combining Diacritical Marks. An important advance conceived by Unicode in designing the UCS and related algorithms for handling text was the introduction of combining diacritic marks. By providing accents that can combine with any letter character, the Unicode and the UCS reduce significantly the number of characters needed. While the UCS also includes precomposed characters, these were included primarily to facilitate support within UCS for non-Unicode text processing systems. * Punctuation. Along with unifying diacritical marks, the UCS also sought to unify punctuation across scripts. Many scripts also contain punctuation, however, when that punctuation has no similar semantics in other scripts. * Symbols. Many mathematics, technical, geometrical and other symbols are included within the UCS. This provides distinct symbols with their own code point or character rather than relying on switching fonts to provide symbolic glyphs. ** Currency. ** Letterlike. These symbols appear like combinations of many common Latin scripts letters such as ℅. Unicode designates many of the letterlike symbols as compatibility characters usually because they can be in plain text by substituting glyphs for a composing sequence of characters: for example substituting the glyph ℅ for the composed sequence of characters c/o. ** Number Forms. Number forms primarily consist of precomposed fractions and Roman numerals. Like other areas of composing sequences of characters, the Unicode approach prefers the flexibility of composing fractions by combining characters together. In this case to create fractions, one combines numbers with the fraction slash character (U+2044). As an example of the flexibility this approach provides, there are nineteen precomposed fraction characters included within the UCS. However, there are an infinity of possible fractions. By using composing characters the infinity of fractions is handled by 11 characters (0-9 and the fraction slash). No character set could include code points for every precomposed fraction. Ideally a text system should present the same glyphs for a fraction whether it is one of the precomposed fractions (such as ⅓) or a composing sequence of characters (such as 1⁄3). However, web browsers are not typically that sophisticated with Unicode and text handling. Doing so ensures that precomposed fractions and combining sequence fractions will appear compatible next to each other. ** Arrows. ** Mathematical. ** Geometric Shapes. ** Legacy Computing. ** Control Pictures Graphical representations of many control characters. ** Box Drawing. ** Block Elements. ** Braille Patterns. ** Optical Character Recognition. ** Technical. ** Dingbats. ** Miscellaneous Symbols. ** Emoticons. ** Symbols and Pictographs. ** Alchemical Symbols. ** Game Pieces (chess, checkers, go, dice, dominoes, mahjong, playing cards, and many others). ** Chess Symbols **
Tai Xuan Jing The text ''Tài Xuán Jīng'' ("Canon of Supreme Mystery", ) is a guide for divination composed by the Confucian writer Yang Xiong (53 BCE – 18 CE). The first draft of this work was completed in 2 BCE (in the decade before the fall of the Wes ...
. ** Yijing Hexagram Symbols. * CJK. Devoted to ideographs and other characters to support languages in China, Japan, Korea (CJK), Taiwan, Vietnam, and Thailand. ** Radicals and Strokes. ** Ideographs. By far the largest portion of the UCS is devoted to ideographs used in languages of Eastern Asia. While the glyph representation of these ideographs have diverged in the languages that use them, the UCS unifies these
Han characters Chinese characters () are logograms developed for the writing of Chinese. In addition, they have been adapted to write other East Asian languages, and remain a key component of the Japanese writing system where they are known as ''kanji' ...
in what Unicode refers to as Unihan (for Unified Han). With Unihan, the text layout software must work together with the available fonts and these Unicode characters to produce the appropriate glyph for the appropriate language. Despite unifying these characters, the UCS still includes over 97,000 Unihan ideographs. * Musical Notation. * Duployan shorthands. * Sutton SignWriting. * Compatibility Characters. Several blocks in the UCS are devoted almost entirely to compatibility characters. Compatibility characters are those included for support of legacy text handling systems that do not make a distinction between character and glyph the way Unicode does. For example, many Arabic letters are represented by a different glyph when the letter appears at the end of a word than when the letter appears at the beginning of a word. Unicode's approach prefers to have these letters mapped to the same character for ease of internal machine text processing and storage. To complement this approach, the text software must select different glyph variants for display of the character based on its context. Over 4000 characters are included for such compatibility reasons. * Control Characters. * Surrogates. The UCS includes 2048 code points in the Basic Multilingual Plane (BMP) for surrogate code point pairs. Together these surrogates allow any code point in the sixteen other planes to be addressed by using two surrogate code points. This provides a simple built-in method for encoding the 20.1 bit UCS within a 16 bit encoding such as UTF-16. In this way UTF-16 can represent any character within the BMP with a single 16-bit byte. Characters outside the BMP are then encoded using two 16-bit bytes (4 octets total) using the surrogate pairs. * Private Use. The consortium provides several private use blocks and planes that can be assigned characters within various communities, as well as operating system and font vendors. * . The consortium guarantees certain code points will never be assigned a character and calls these code points. The last two code points of each plane (ending in FE and FF ) are such code points. There are a few others interspersed throughout the Basic Multilingual Plane, the first plane.


Special-purpose characters

Unicode codifies over a hundred thousand characters. Most of those represent graphemes for processing as linear text. Some, however, either do not represent graphemes, or, as graphemes, require exceptional treatment. Unlike the ASCII control characters and other characters included for legacy round-trip capabilities, these other special-purpose characters endow plain text with important semantics. Some special characters can alter the layout of text, such as the zero-width joiner and zero-width non-joiner, while others do not affect text layout at all, but instead affect the way text strings are collated, matched or otherwise processed. Other special-purpose characters, such as the
mathematical invisibles Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, generally have no effect on text rendering, though sophisticated text layout software may choose to subtly adjust spacing around them. Unicode does not specify the division of labor between font and text layout software (or "engine") when rendering Unicode text. Because the more complex font formats, such as
OpenType OpenType is a format for scalable computer fonts. It was built on its predecessor TrueType, retaining TrueType's basic structure and adding many intricate data structures for prescribing typographic behavior. OpenType is a registered trademark o ...
or
Apple Advanced Typography Apple Advanced Typography (AAT) is Apple Inc.'s computer technology for advanced font rendering, supporting internationalization and complex features for typographers, a successor to Apple's little-used QuickDraw GX font technology of the mid-1 ...
, provide for contextual substitution and positioning of glyphs, a simple text layout engine might rely entirely on the font for all decisions of glyph choice and placement. In the same situation a more complex engine may combine information from the font with its own rules to achieve its own idea of best rendering. To implement all recommendations of the Unicode specification, a text engine must be prepared to work with fonts of any level of sophistication, since contextual substitution and positioning rules do not exist in some font formats and are optional in the rest. The
fraction slash The slash is the oblique slanting line punctuation mark . Also known as a stroke, a solidus or several other historical or technical names including oblique and virgule. Once used to mark periods and commas, the slash is now used to represen ...
is an example: complex fonts may or may not supply positioning rules in the presence of the fraction slash character to create a fraction, while fonts in simple formats cannot.


Byte order mark

When appearing at the head of a text file or stream, the
byte order mark The byte order mark (BOM) is a particular usage of the special Unicode character, , whose appearance as a magic number at the start of a text stream can signal several things to a program reading the text: * The byte order, or endianness, of th ...
(BOM) U+FEFF hints at the encoding form and its byte order. If the stream's first byte is 0xFE and the second 0xFF, then the stream's text is not likely to be encoded in
UTF-8 UTF-8 is a variable-width encoding, variable-length character encoding used for electronic communication. Defined by the Unicode Standard, the name is derived from ''Unicode'' (or ''Universal Coded Character Set'') ''Transformation Format 8-bit'' ...
, since those bytes are invalid in UTF-8. It is also not likely to be
UTF-16 UTF-16 (16-bit computing, 16-bit Unicode Transformation Format) is a character encoding capable of encoding all 1,112,064 valid code points of Unicode (in fact this number of code points is dictated by the design of UTF-16). The encoding is variab ...
in
little-endian In computing, endianness, also known as byte sex, is the order or sequence of bytes of a word of digital data in computer memory. Endianness is primarily expressed as big-endian (BE) or little-endian (LE). A big-endian system stores the most si ...
byte order because 0xFE, 0xFF read as a 16-bit little endian word would be U+FFFE, which is meaningless. The sequence also has no meaning in any arrangement of
UTF-32 UTF-32 (32-bit Unicode transformation format, Unicode Transformation Format) is a fixed-length Character encoding, encoding used to encode Unicode code points that uses exactly 32 bits (four bytes) per code point (but a number of leading bits must ...
encoding, so, in summary, it serves as a fairly reliable indication that the text stream is encoded as UTF-16 in
big-endian In computing, endianness, also known as byte sex, is the order or sequence of bytes of a word of digital data in computer memory. Endianness is primarily expressed as big-endian (BE) or little-endian (LE). A big-endian system stores the most sig ...
byte order. Conversely, if the first two bytes are 0xFF, 0xFE, then the text stream may be assumed to be encoded as UTF-16LE because, read as a 16-bit little-endian value, the bytes yield the expected 0xFEFF byte order mark. This assumption becomes questionable, however, if the next two bytes are both 0x00; either the text begins with a null character (U+0000), or the correct encoding is actually UTF-32LE, in which the full 4-byte sequence FF FE 00 00 is one character, the BOM. The UTF-8 sequence corresponding to U+FEFF is 0xEF, 0xBB, 0xBF. This sequence has no meaning in other Unicode encoding forms, so it may serve to indicate that that stream is encoded as UTF-8. The Unicode specification does not require the use of byte order marks in text streams. It further states that they should not be used in situations where some other method of signaling the encoding form is already in use.


Mathematical invisibles

Primarily for mathematics, the Invisible Separator (U+2063) provides a separator between characters where punctuation or space may be omitted such as in a two-dimensional index like i⁣j. Invisible Times (U+2062) and Function Application (U+2061) are useful in mathematics text where the multiplication of terms or the application of a function is implied without any glyph indicating the operation. Unicode 5.1 introduces the Mathematical Invisible Plus character as well (U+2064) which may indicate that an integral number followed by a fraction should denote their sum, but not their product.


Fraction slash

The fraction slash character (U+2044) has special behavior in the Unicode Standard: (section 6.2, Other Punctuation)
The standard form of a fraction built using the fraction slash is defined as follows: any sequence of one or more decimal digits (General Category = Nd), followed by the fraction slash, followed by any sequence of one or more decimal digits. Such a fraction should be displayed as a unit, such as ¾. If the displaying software is incapable of mapping the fraction to a unit, then it can also be displayed as a simple linear sequence as a fallback (for example, 3/4). If the fraction is to be separated from a previous number, then a space can be used, choosing the appropriate width (normal, thin, zero width, and so on). For example, 1 + ZERO WIDTH SPACE + 3 + FRACTION SLASH + 4 is displayed as 1¾.
By following this Unicode recommendation, text processing systems yield sophisticated symbols from plain text alone. Here the presence of the fraction slash character instructs the layout engine to synthesize a fraction from all consecutive digits preceding and following the slash. In practice, results vary because of the complicated interplay between fonts and layout engines. Simple text layout engines tend not to synthesize fractions at all, and instead draw the glyphs as a linear sequence as described in the Unicode fallback scheme. More sophisticated layout engines face two practical choices: they can follow Unicode's recommendation, or they can rely on the font's own instructions for synthesizing fractions. By ignoring the font's instructions, the layout engine can guarantee Unicode's recommended behavior. By following the font's instructions, the layout engine can achieve better
typography Typography is the art and technique of arranging type to make written language legible, readable and appealing when displayed. The arrangement of type involves selecting typefaces, point sizes, line lengths, line-spacing ( leading), and ...
because placement and shaping of the digits will be tuned to that particular font at that particular size. The problem with following the font's instructions is that the simpler font formats have no way to specify fraction synthesis behavior. Meanwhile, the more complex formats do not require the font to specify fraction synthesis behavior and therefore many do not. Most fonts of complex formats can instruct the layout engine to replace a plain text sequence such as "1⁄2" with the precomposed "½" glyph. But because many of them will not issue instructions to synthesize fractions, a plain text string such as "221⁄225" may well render as 22½25 (with the ½ being the substituted precomposed fraction, rather than synthesized). In the face of problems like this, those who wish to rely on the recommended Unicode behavior should choose fonts known to synthesize fractions or text layout software known to produce Unicode's recommended behavior regardless of font.


Bidirectional neutral formatting

Writing direction is the direction glyphs are placed on the page in relation to forward progression of characters in the Unicode string. English and other languages of Latin script have left-to-right writing direction. Several major writing scripts, such as
Arabic Arabic (, ' ; , ' or ) is a Semitic languages, Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C ...
and
Hebrew Hebrew (; ; ) is a Northwest Semitic language of the Afroasiatic language family. Historically, it is one of the spoken languages of the Israelites and their longest-surviving descendants, the Jews and Samaritans. It was largely preserved ...
, have right-to-left writing direction. The Unicode specification assigns a ''directional type'' to each character to inform text processors how sequences of characters should be ordered on the page. While lexical characters (that is, letters) are normally specific to a single writing script, some symbols and punctuation marks are used across many writing scripts. Unicode could have created duplicate symbols in the repertoire that differ only by directional type, but chose instead to unify them and assign them a neutral directional type. They acquire direction at render time from adjacent characters. Some of these characters also have a ''bidi-mirrored'' property indicating the glyph should be rendered in mirror-image when used in right-to-left text. The render-time directional type of a neutral character can remain ambiguous when the mark is placed on the boundary between directional changes. To address this, Unicode includes characters that have strong directionality, have no glyph associated with them, and are ignorable by systems that do not process bidirectional text: * Arabic letter mark (U+061C) * Left-to-right mark (U+200E) * Right-to-left mark (U+200F) Surrounding a bidirectionally neutral character by the left-to-right mark will force the character to behave as a left-to-right character while surrounding it by the right-to-left mark will force it to behave as a right-to-left character. The behavior of these characters is detailed in Unicode's Bidirectional Algorithm.


Bidirectional general formatting

While Unicode is designed to handle multiple languages, multiple writing systems and even text that flows either left-to-right or right-to-left with minimal author intervention, there are special circumstances where the mix of bidirectional text can become intricate—requiring more author control. For these circumstances, Unicode includes five other characters to control the complex embedding of left-to-right text within right-to-left text and vice versa: * Left-to-right embedding (U+202A) * Right-to-left embedding (U+202B) * Pop directional formatting (U+202C) * Left-to-right override (U+202D) * Right-to-left override (U+202E) * Left-to-right isolate (U+2066) * Right-to-left isolate (U+2067) * First strong isolate (U+2068) * Pop directional isolate (U+2069)


Interlinear annotation characters

* Interlinear Annotation Anchor (U+FFF9) * Interlinear Annotation Separator (U+FFFA) * Interlinear Annotation Terminator (U+FFFB)


Script-specific

* Prefixed format control ** Arabic Number Sign (U+0600) ** Arabic Sign Sanah (U+0601) ** Arabic Footnote Marker (U+0602) ** Arabic Sign Safha (U+0603) ** Arabic Sign Samvat (U+0604) ** Arabic Number Mark Above (U+0605) ** Arabic End of Ayah (U+06DD) **
Syriac Abbreviation Mark The Syriac Abbreviation Mark is a Unicode Control character (U+070F) that forms part of the Syriac script block. In Syriac, words are sometimes written in an abbreviated form, omitting some of the last letters. In such cases, a special overline is ...
(U+070F) ** Arabic Pound Mark Above (U+0890) ** Arabic Piastre Mark Above (U+0891) ** Kaithi Number Sign (U+110BD) ** Kaithi Number Sign Above (U+110CD) * Egyptian Hieroglyphs ** Egyptian Hieroglyph Vertical Joiner (U+13430) ** Egyptian Hieroglyph Horizontal Joiner (U+13431) ** Egyptian Hieroglyph Insert At Top Start (U+13432) ** Egyptian Hieroglyph Insert At Bottom Start (U+13433) ** Egyptian Hieroglyph Insert At Top End (U+13434) ** Egyptian Hieroglyph Insert At Bottom End (U+13435) ** Egyptian Hieroglyph Overlay Middle (U+13436) ** Egyptian Hieroglyph Begin Segment (U+13437) ** Egyptian Hieroglyph End Segment (U+13438) ** Egyptian Hieroglyph Insert At Middle (U+13439) ** Egyptian Hieroglyph Insert At Top (U+1343A) ** Egyptian Hieroglyph Insert At Bottom (U+1343B) ** Egyptian Hieroglyph Begin Enclosure (U+1343C) ** Egyptian Hieroglyph End Enclosure (U+1343D) ** Egyptian Hieroglyph Begin Walled Enclosure (U+1343E) ** Egyptian Hieroglyph End Walled Enclosure (U+1343F) * Brahmi ** Brahmi Number Joiner (U+1107F) * Brahmi-derived script dead-character formation (
Virama Virama ( ्) is a Sanskrit phonological concept to suppress the inherent vowel that otherwise occurs with every consonant letter, commonly used as a generic term for a codepoint in Unicode, representing either # halanta, hasanta or explicit virā ...
and similar diacritics) ** Devanagari Sign Virama (U+094D) ** Bengali Sign Virama (U+09CD) ** Gurmukhi Sign Virama (U+0A4D) ** Gujarati Sign Virama (U+0ACD) ** Oriya Sign Virama (U+0B4D) ** Tamil Sign Virama (U+0BCD) ** Telugu Sign Virama (U+0C4D) ** Kannada Sign Virama (U+0CCD) ** Malayalam Sign Vertical Bar Virama (U+0D3B) ** Malayalam Sign Circular Virama (U+0D3C) ** Malayalam Sign Virama (U+0D4D) ** Sinhala Sign Al-Lakuna (U+0DCA) ** Thai Character Phinthu (U+0E3A) ** Thai Character Yamakkan (U+0E4E) ** Lao Sign Pali Virama (U+0EBA) ** Myanmar Sign Virama (U+1039) ** Tagalog Sign Virama (U+1714) ** Tagalog Sign Pamudpod (U+1715) ** Hanunoo Sign Pamudpod (U+1734) ** Khmer Sign Viriam (U+17D1) ** Khmer Sign Coeng (U+17D2) ** Tai Tham Sign Sakot (U+1A60) ** Tai Tham Sign Ra Haam (U+1A7A) ** Balinese Adeg Adeg (U+1B44) ** Sundanese Sign Pamaaeh (U+1BAA) ** Sundanese Sign Virama (U+1BAB) ** Batak Pangolat (U+1BF2) ** Batak Panongonan (U+1BF3) ** Syloti Nagri Sign Hasanta (U+A806) ** Syloti Nagri Sign Alternate Hasanta (U+A82C) ** Saurashtra Sign Virama (U+A8C4) ** Rejang Virama (U+A953) ** Javanese Pangkon (U+A9C0) ** Meetei Mayek Virama (U+AAF6) ** Kharoshthi Virama (U+10A3F) ** Brahmi Virama (U+11046) ** Brahmi Sign Old Tamil Virama (U+11070) ** Kaithi Sign Virama (U+110B9) ** Chakma Virama (U+11133) ** Sharada Sign Virama (U+111C0) ** Khojki Sign Virama (U+11235) ** Khudawadi Sign Virama (U+112EA) ** Grantha Sign Virama (U+1134D) ** Newa Sign Virama (U+11442) ** Tirhuta Sign Virama (U+114C2) ** Siddham Sign Virama (U+115BF) ** Modi Sign Virama (U+1163F) ** Takri Sign Virama (U+116B6) ** Ahom Sign Killer (U+1172B) ** Dogra Sign Virama (U+11839) ** Dives Akuru Sign Halanta (U+1193D) ** Dives Akuru Virama (U+1193E) ** Nandinagari Sign Virama (U+119E0) ** Zanabazar Square Sign Virama (U+11A34) ** Zanabazar Square Subjoiner (U+11A47) ** Soyombo Subjoiner (U+11A99) ** Bhaiksuki Sign Virama (U+11C3F) ** Masaram Gondi Sign Halanta (U+11D44) ** Masaram Gondi Virama (U+11D45) ** Gunjala Gondi Virama (U+11D97) ** Kawi Sign Killer (U+11F41) ** Kawi Conjoiner (U+11F42) * Historical Viramas with other functions ** Tibetan Mark Halanta (U+0F84) ** Myanmar Sign Asat (U+103A) ** Limbu Sign Sa-I (U+193B) ** Meetei Mayek Apun Iyek (U+ABED) ** Chakma Maayyaa (U+11134) * Mongolian Variation Selectors ** Mongolian Free Variation Selector One (U+180B) ** Mongolian Free Variation Selector Two (U+180C) ** Mongolian Free Variation Selector Three (U+180D) ** Mongolian Vowel Separator (U+180E) * Generic Variation Selectors ** Variation Selector-1 through -16 (U+FE00–U+FE0F) ** Variation Selector-17 through -256 (U+E0100–U+E01EF) * Tag characters (U+E0001 and U+E0020–U+E007F) * Tifinagh ** Tifinagh Consonant Joiner (U+2D7F) * Ogham ** Ogham Space Mark (U+1680) * Ideographic ** Ideographic variation indicator (U+303E) ** Ideographic Description (U+2FF0–U+2FFB) * Musical Format Control ** Musical Symbol Begin Beam (U+1D173) ** Musical Symbol End Beam (U+1D174) ** Musical Symbol Begin Tie (U+1D175) ** Musical Symbol End Tie (U+1D176) ** Musical Symbol Begin Slur (U+1D177) ** Musical Symbol End Slur (U+1D178) ** Musical Symbol Begin Phrase (U+1D179) ** Musical Symbol End Phrase (U+1D17A) * Shorthand Format Control ** Shorthand Format Letter Overlap (U+1BCA0) ** Shorthand Format Continuing Overlap (U+1BCA1) ** Shorthand Format Down Step (U+1BCA2) ** Shorthand Format Up Step (U+1BCA3) * Deprecated Alternate Formatting ** Inhibit Symmetric Swapping (U+206A) ** Activate Symmetric Swapping (U+206B) ** Inhibit Arabic Form Shaping (U+206C) ** Activate Arabic Form Shaping (U+206D) ** National Digit Shapes (U+206E) ** Nominal Digit Shapes (U+206F)


Others

* Object Replacement Character (U+FFFC) * Replacement Character (U+FFFD)


Characters vs code points

The term "character" is not well defined, and what we are referring to most of the time is the
grapheme In linguistics, a grapheme is the smallest functional unit of a writing system. The word ''grapheme'' is derived and the suffix ''-eme'' by analogy with ''phoneme'' and other names of emic units. The study of graphemes is called ''graphemics' ...
. A grapheme is represented visually by its
glyph A glyph () is any kind of purposeful mark. In typography, a glyph is "the specific shape, design, or representation of a character". It is a particular graphical representation, in a particular typeface, of an element of written language. A g ...
. The
typeface A typeface (or font family) is the design of lettering that can include variations in size, weight (e.g. bold), slope (e.g. italic), width (e.g. condensed), and so on. Each of these variations of the typeface is a font. There are list of type ...
(often erroneously referred to as
font In metal typesetting, a font is a particular size, weight and style of a typeface. Each font is a matched set of type, with a piece (a "sort") for each glyph. A typeface consists of a range of such fonts that shared an overall design. In mod ...
) used can depict visual variations of the same character. It is possible that two different graphemes can have the exact same glyph or are visually so close that the average reader cannot tell them apart. A grapheme is almost always represented by one code point, for example the LATIN CAPITAL LETTER A is represented by only code point U+0041. The grapheme LATIN CAPITAL A WITH DIAERESIS Ä is an example where a character can be represented by more than one code point. It can be U+00C4, or U+0041U+0308. U+0041 is the familiar A and U+0308 is the COMBINING DIAERESIS ̈, a
combining diacritical mark In digital typography, combining characters are characters that are intended to modify other characters. The most common combining characters in the Latin script are the combining diacritical marks (including combining accents). Unicode also ...
. When a combining mark is adjacent to a non-combining mark code point, text rendering applications should superimpose the combining mark onto the glyph represented by the other code point to form a grapheme according to a set of rules. The word BÄM would therefore be three graphemes. It may be made up of three code points or more depending on how the characters are actually composed.


Whitespace, joiners, and separators

Unicode provides a list of characters it deems whitespace characters for interoperability support. Software Implementations and other standards may use the term to denote a slightly different set of characters. For example, Java does not consider or to be whitespace, even though Unicode does. Whitespace characters are characters typically designated for programming environments. Often they have no syntactic meaning in such programming environments and are ignored by the machine interpreters. Unicode designates the legacy control characters U+0009 through U+000D and U+0085 as whitespace characters, as well as all characters whose General Category property value is Separator. There are 25 total whitespace characters as of Unicode 15.0.


Grapheme joiners and non-joiners

The
zero-width joiner The zero-width joiner (ZWJ, ) is a non-printing character used in the computerized typesetting of writing systems in which the shape or positioning of a grapheme depends on its relation to other graphemes (complex scripts), such as the Arabic s ...
(U+200D) and
zero-width non-joiner The zero-width non-joiner (ZWNJ) is a non-printing character used in the computerization of writing systems that make use of ligatures. When placed between two characters that would otherwise be connected into a ligature, a ZWNJ causes them to ...
(U+200C) control the joining and ligation of glyphs. The joiner does not cause characters that would not otherwise join or ligate to do so, but when paired with the non-joiner these characters can be used to control the joining and ligating properties of the surrounding two joining or ligating characters. The Combining Grapheme Joiner (U+034F) is used to distinguish two base characters as one common base or digraph, mostly for underlying text processing, collation of strings, case folding and so on.


Word joiners and separators

The most common word separator is a space (U+0020). However, there are other word joiners and separators that also indicate a break between words and participate in line-breaking algorithms. The No-Break Space (U+00A0) also produces a baseline advance without a glyph but inhibits rather than enabling a line-break. The Zero Width Space (U+200B) allows a line-break but provides no space: in a sense joining, rather than separating, two words. Finally, the Word Joiner (U+2060) inhibits line breaks and also involves none of the white space produced by a baseline advance.


Other separators

* Line Separator (U+2028) * Paragraph Separator (U+2029) These provide Unicode with native paragraph and line separators independent of the legacy encoded ASCII control characters such as carriage return (U+000A), linefeed (U+000D), and Next Line (U+0085). Unicode does not provide for other ASCII formatting control characters which presumably then are not part of the Unicode plain text processing model. These legacy formatting control characters include Tab (U+0009), Line Tabulation or Vertical Tab (U+000B), and Form Feed (U+000C) which is also thought of as a page break.


Spaces

The space character (U+0020) typically input by the space bar on a keyboard serves semantically as a word separator in many languages. For legacy reasons, the UCS also includes spaces of varying sizes that are compatibility equivalents for the space character. While these spaces of varying width are important in typography, the Unicode processing model calls for such visual effects to be handled by rich text, markup and other such protocols. They are included in the Unicode repertoire primarily to handle lossless roundtrip transcoding from other character set encodings. These spaces include: # En Quad (U+2000) # Em Quad (U+2001) # En Space (U+2002) # Em Space (U+2003) # Three-Per-Em Space (U+2004) # Four-Per-Em Space (U+2005) # Six-Per-Em Space (U+2006) # Figure Space (U+2007) # Punctuation Space (U+2008) # Thin Space (U+2009) # Hair Space (U+200A) # Medium Mathematical Space (U+205F) Aside from the original ASCII space, the other spaces are all compatibility characters. In this context this means that they effectively add no semantic content to the text, but instead provide styling control. Within Unicode, this non-semantic styling control is often referred to as rich text and is outside the thrust of Unicode's goals. Rather than using different spaces in different contexts, this styling should instead be handled through intelligent text layout software. Three other writing-system-specific word separators are: * Mongolian Vowel Separator (U+180E) * Ideographic Space (U+3000): behaves as an ideographic separator and generally rendered as white space of the same width as an ideograph. * Ogham Space Mark (U+1680): this character is sometimes displayed with a glyph and other times as only white space.


Line-break control characters

Several characters are designed to help control line-breaks either by discouraging them (no-break characters) or suggesting line breaks such as the soft hyphen (U+00AD) (sometimes called the "shy hyphen"). Such characters, though designed for styling, are probably indispensable for the intricate types of line-breaking they make possible. ;Break inhibiting # Non-breaking hyphen (U+2011) # No-break space (U+00A0) # Tibetan Mark Delimiter Tsheg Bstar (U+0F0C) # Narrow no-break space (U+202F) The break inhibiting characters are meant to be equivalent to a character sequence wrapped in the Word Joiner U+2060. However, the Word Joiner may be appended before or after any character that would allow a line-break to inhibit such line-breaking. ;Break enabling # Soft hyphen (U+00AD) # Tibetan Mark Intersyllabic Tsheg (U+0F0B) # Zero-width space (U+200B) Both the break inhibiting and break enabling characters participate with other punctuation and whitespace characters to enable text imaging systems to determine line breaks within the Unicode Line Breaking Algorithm.


Types of code point

All code points given some kind of purpose or use are considered designated code points. Of those, they may be assigned to an abstract character, or otherwise designated for some other purpose.


Assigned characters

The majority of code points in actual use have been assigned to abstract characters. This includes private-use characters, which though not formally designated by the Unicode standard for a particular purpose, require a sender and recipient to have agreed in advance how they should be interpreted for meaningful information interchange to take place.


Private-use characters

The UCS includes 137,468 private-use characters, which are code points for private use spread across three different blocks, each called a ''Private Use Area'' (PUA). The Unicode standard recognizes code points within PUAs as legitimate Unicode character codes, but does not assign them any (abstract) character. Instead, individuals, organizations, software vendors, operating system vendors, font vendors and communities of end-users are free to use them as they see fit. Within closed systems, characters in the PUA can operate unambiguously, allowing such systems to represent characters or glyphs not defined in Unicode. In public systems their use is more problematic, since there is no registry and no way to prevent several organizations from adopting the same code points for different purposes. One example of such a conflict is
Apple An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple fruit tree, trees are agriculture, cultivated worldwide and are the most widely grown species in the genus ''Malus''. The tree originated in Central Asia, wh ...
's use of
U+F8FF In Unicode, a Private Use Area (PUA) is a range of code points that, by definition, will not be assigned characters by the Unicode Consortium. Three private use areas are defined: one in the Basic Multilingual Plane (), and one each in, and nearl ...
for the Apple logo, versus the
ConScript Unicode Registry The ConScript Unicode Registry is a discontinued volunteer project to coordinate the assignment of code points in the Unicode Private Use Areas (PUA) for the encoding of artificial scripts including those for constructed languages. It was founded by ...
's use of U+F8FF as in the
Klingon script The Klingon scripts are fictional alphabetic scripts used in the ''Star Trek'' movies and television shows to write the Klingon language. In Marc Okrand's ''The Klingon Dictionary'', the Klingon script is called ', but no information is given ...
. The Basic Multilingual Plane (Plane 0) contains 6,400 private-user characters in the eponymously named PUA ''Private Use Area'', which ranges from U+E000 to U+F8FF. The Private Use Planes, Plane 15 and Plane 16, each have their own PUAs of 65,534 private-use characters (with the final two code points of each plane being ). These are ''Supplementary Private Use Area-A'', which ranges from U+F0000 to U+FFFFD, and ''Supplementary Private Use Area-B'', which ranges from U+100000 to U+10FFFD. PUAs are a concept inherited from certain Asian encoding systems. These systems had private use areas to encode what the Japanese call ''
gaiji are the logographic Chinese characters taken from the Chinese script and used in the writing of Japanese. They were made a major part of the Japanese writing system during the time of Old Japanese and are still used, along with the subseque ...
'' (rare characters not normally found in fonts) in application-specific ways.


Surrogates

The UCS uses surrogates to address characters outside the initial
Basic Multilingual Plane In the Unicode standard, a plane is a continuous group of 65,536 (216) code points. There are 17 planes, identified by the numbers 0 to 16, which corresponds with the possible values 00–1016 of the first two positions in six position hexadecimal ...
without resorting to more-than-16-bit byte representations. There are 1024 "high" surrogates (D800–DBFF) and 1024 "low" surrogates (DC00–DFFF). By combining a pair of surrogates, the remaining characters in all the other planes can be addressed (1024 × 1024 = 1048576 code points in the other 16 planes). In
UTF-16 UTF-16 (16-bit computing, 16-bit Unicode Transformation Format) is a character encoding capable of encoding all 1,112,064 valid code points of Unicode (in fact this number of code points is dictated by the design of UTF-16). The encoding is variab ...
, they must always appear in pairs, as a high surrogate followed by a low surrogate, thus using 32 bits to denote one code point. A surrogate pair denotes the code point :1000016 + (''H'' - D80016) × 40016 + (''L'' - DC0016) where ''H'' and ''L'' are the numeric values of the high and low surrogates respectively. Since high surrogate values in the range DB80–DBFF always produce values in the Private Use planes, the high surrogate range can be further divided into (normal) high surrogates (D800–DB7F) and "high private use surrogates" (DB80–DBFF). Isolated surrogate code points have no general interpretation; consequently, no character code charts or names lists are provided for this range. In the
Python programming language Python is a high-level, general-purpose programming language. Its design philosophy emphasizes code readability with the use of significant indentation. Python is dynamically-typed and garbage-collected. It supports multiple programming p ...
, individual surrogate codes are used to embed undecodable bytes in Unicode strings.


The unhyphenated term "" refers to 66 code points (labeled ) permanently reserved for internal use, and therefore guaranteed to never be assigned to a character. Each of the 17 planes has its two ending code points set aside as . So, are: U+FFFE and U+FFFF on the BMP, U+1FFFE and U+1FFFF on Plane 1, and so on, up to U+10FFFE and U+10FFFF on Plane 16, for a total of 34 code points. In addition, there is a contiguous range of another 32 code points in the BMP: U+FDD0..U+FDEF. Software implementations are therefore free to use these code points for internal use. One particularly useful example of a is the code point U+FFFE. This code point has the reverse UTF-16/UCS-2 byte sequence of the
byte order mark The byte order mark (BOM) is a particular usage of the special Unicode character, , whose appearance as a magic number at the start of a text stream can signal several things to a program reading the text: * The byte order, or endianness, of th ...
(U+FEFF). If a stream of text contains this , this is a good indication the text has been interpreted with the incorrect
endianness In computing, endianness, also known as byte sex, is the order or sequence of bytes of a word of digital data in computer memory. Endianness is primarily expressed as big-endian (BE) or little-endian (LE). A big-endian system stores the most sig ...
. Versions of the Unicode standard from 3.1.0 to 6.3.0 claimed that "should never be interchanged"
Corrigendum #9
of the standard later stated that this was leading to "inappropriate over-rejection", clarifying that "[] are not illegal in interchange nor do they cause ill-formed Unicode text", and removing the original claim.


Reserved code points

All other code points, being those not designated, are referred to as being reserved. These code points may be assigned for a particular use in future versions of the Unicode standard.


Characters, grapheme clusters and glyphs

Whereas many other character sets assign a character for every possible glyph representation of the character, Unicode seeks to treat characters separately from glyphs. This distinction is not always unambiguous, however a few examples will help illustrate the distinction. Often two characters may be combined typographically to improve the readability of the text. For example, the three letter sequence "ffi" may be treated as a single glyph. Other character sets would often assign a code point to this glyph in addition to the individual letters: "f" and "i". In addition, Unicode approaches
diacritic A diacritic (also diacritical mark, diacritical point, diacritical sign, or accent) is a glyph added to a letter or to a basic glyph. The term derives from the Ancient Greek (, "distinguishing"), from (, "to distinguish"). The word ''diacriti ...
modified letters as separate characters that, when rendered, become a single glyph. For example, an "o" with diaeresis: " ö". Traditionally, other character sets assigned a unique character code point for each diacritic modified letter used in each language. Unicode seeks to create a more flexible approach by allowing combining diacritic characters to combine with any letter. This has the potential to significantly reduce the number of active code points needed for the character set. As an example, consider a language that uses the Latin script and combines the diaeresis with the upper- and lower-case letters "a", "o", and "u". With the Unicode approach, only the diaeresis diacritic character needs to be added to the character set to use with the Latin letters: "a", "A", "o", "O", "u", and "U": seven characters in all. A legacy character sets needs to add six precomposed letters with a diaeresis in addition to the six code points it uses for the letters without diaeresis: twelve character code points in total.


Compatibility characters

UCS includes thousands of characters that Unicode designates as compatibility characters. These are characters that were included in UCS in order to provide distinct code points for characters that other character sets differentiate, but would not be differentiated in the Unicode approach to characters. The chief reason for this differentiation was that Unicode makes a distinction between characters and glyphs. For example, when writing English in a
cursive Cursive (also known as script, among other names) is any style of penmanship in which characters are written joined in a flowing manner, generally for the purpose of making writing faster, in contrast to block letters. It varies in functionalit ...
style, the letter "i" may take different forms whether it appears at the beginning of a word, the end of a word, the middle of a word or in isolation. Languages such as
Arabic Arabic (, ' ; , ' or ) is a Semitic languages, Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C ...
written in an Arabic script are always cursive. Each letter has many different forms. UCS includes 730 Arabic form characters that decompose to just 88 unique Arabic characters. However, these additional Arabic characters are included so that text processing software may translate text from other character sets to UCS and back again without any loss of information crucial for non-Unicode software. However, for UCS and Unicode in particular, the preferred approach is to always encode or map that letter to the same character no matter where it appears in a word. Then the distinct forms of each letter are determined by the font and text layout software methods. In this way, the internal memory for the characters remains identical regardless of where the character appears in a word. This greatly simplifies searching, sorting and other text processing operations.


Character properties

Every character in Unicode is defined by a large and growing set of properties. Most of these properties are not part of Universal Character Set. The properties facilitate text processing including collation or sorting of text, identifying words, sentences and graphemes, rendering or imaging text and so on. Below is a list of some of the core properties. There are many others documented in the Unicode Character Database. Unicode provides an online database to interactively query the entire Unicode character repertoire by the various properties.


See also

*
ConScript Unicode Registry The ConScript Unicode Registry is a discontinued volunteer project to coordinate the assignment of code points in the Unicode Private Use Areas (PUA) for the encoding of artificial scripts including those for constructed languages. It was founded by ...
*
Unicode compatibility characters In Unicode and the UCS, a compatibility character is a character that is encoded solely to maintain round-trip convertibility with other, often older, standards. As the Unicode Glossary says: A character that would not have been encoded excep ...


References


External links


Unicode Consortium

decodeunicode.org
Unicode Wiki with all 98884 graphic characters of Unicode 5.0 as gifs, full text search
Unicode Characters by Property
{{Unicode navigation IEC standards Unicode