The Universal Coded Character Set (UCS, Unicode) is a standard set of characters defined by the

international standard An international standard is a technical standard developed by one or more international standards organizations. International standards are available for consideration and use worldwide. The most prominent such organization is the International O ...

ISO The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries. Me ...

/ IEC 10646, ''Information technology — Universal Coded Character Set (UCS)'' (plus amendments to that standard), which is the basis of many

character encoding Character encoding is the process of assigning numbers to graphical character (computing), characters, especially the written characters of human language, allowing them to be stored, transmitted, and transformed using computers. The numerical v ...

s, improving as characters from previously unrepresented writing systems are added. The UCS has over 1.1 million possible

code point A code point, codepoint or code position is a particular position in a Table (database), table, where the position has been assigned a meaning. The table may be one dimensional (a column), two dimensional (like cells in a spreadsheet), three dime ...

s available for use/allocation, but only the first 65,536, which is the Basic Multilingual Plane (BMP), had entered into common use before 2000. This situation began changing when the

People's Republic of China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

(PRC) ruled in 2006 that all software sold in its jurisdiction would have to support

GB 18030 GB 18030 is a Chinese government standard, described as ''Information Technology — Chinese coded character set'' and defines the required language and character support necessary for software in China. GB18030 is the registered Internet n ...

. This required software intended for sale in the PRC to move beyond the BMP. The system deliberately leaves many code points not assigned to characters, even in the BMP. It does this to allow for future expansion or to minimise conflicts with other encoding forms. The original edition of the UCS defined

UTF-16 UTF-16 (16-bit Unicode Transformation Format) is a character encoding that supports all 1,112,064 valid code points of Unicode. The encoding is variable-length as code points are encoded with one or two ''code units''. UTF-16 arose from an earli ...

, an extension of UCS-2, to represent code points outside the BMP. A range of code points in the S (Special) Zone of the BMP remains unassigned to characters. UCS-2 disallows use of code values for these code points, but UTF-16 allows their use in pairs. Unicode also adopted UTF-16, but in Unicode terminology, the high-half zone elements become "high surrogates" and the low-half zone elements become "low surrogates". Another encoding, UTF-32 (previously named UCS-4), uses four bytes (total 32 bits) to encode a single character of the codespace. UTF-32 thereby permits a binary representation of every code point (as of year 2024) in the APIs, and software applications.

History

The

International Organization for Standardization The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries. M ...

(ISO) set out to compose the universal character set in 1989, and published the draft of ISO 10646 in 1990.

Hugh McGregor Ross Hugh McGregor Ross (31 August 1917 – 1 September 2014) was an early pioneer in the history of British computing. He was employed by Ferranti from the mid-1960s, where he worked on the Ferranti Pegasus, Pegasus Vacuum tube, thermionic valve ...

was one of its principal architects. This work happened independently of the development of the

Unicode Unicode or ''The Unicode Standard'' or TUS is a character encoding standard maintained by the Unicode Consortium designed to support the use of text in all of the world's writing systems that can be digitized. Version 16.0 defines 154,998 Char ...

standard, which had been in development since 1987 by

Xerox Xerox Holdings Corporation (, ) is an American corporation that sells print and electronic document, digital document products and services in more than 160 countries. Xerox was the pioneer of the photocopier market, beginning with the introduc ...

and

Apple An apple is a round, edible fruit produced by an apple tree (''Malus'' spp.). Fruit trees of the orchard or domestic apple (''Malus domestica''), the most widely grown in the genus, are agriculture, cultivated worldwide. The tree originated ...

. The original ISO 10646 draft differed markedly from the current standard. It defined: * 128 groups of * 256 planes of * 256 rows of * 256 cells, for an apparent total of 2,147,483,648 characters, but actually the standard could code only 679,477,248 characters, as the policy forbade byte values of

C0 and C1 control codes The C0 and C1 control code or control character sets define control codes for use in text by computer systems that use ASCII and derivatives of ASCII. The codes represent additional information about the text, such as the position of a cursor, ...

(0x00 to 0x1F and 0x80 to 0x9F, in

hexadecimal Hexadecimal (also known as base-16 or simply hex) is a Numeral system#Positional systems in detail, positional numeral system that represents numbers using a radix (base) of sixteen. Unlike the decimal system representing numbers using ten symbo ...

notation) in any one of the four bytes specifying a group, plane, row and cell. The Latin capital letter A, for example, had a location in group 0x20, plane 0x20, row 0x20, cell 0x41. One could code the characters of this primordial ISO/IEC 10646 standard in one of three ways: # UCS-4, four bytes for every character, enabling the simple encoding of all characters; # UCS-2, two bytes for every character, enabling the encoding of the first plane, 0x20, the Basic Multilingual Plane, containing the first 36,864 codepoints, straightforwardly, and other planes and groups by switching to them with ISO/IEC 2022 escape sequences; # UTF-1, which encodes all the characters in sequences of bytes of varying length (1 to 5 bytes, each of which contain no control codes). In 1990, therefore, two initiatives for a universal character set existed:

, with 16 bits for every character (65,536 possible characters), and ISO/IEC 10646. The software companies refused to accept the complexity and size requirement of the ISO standard and were able to convince a number of ISO National Bodies to vote against it. ISO officials realised they could not continue to support the standard in its current state and negotiated the unification of their standard with Unicode. Two changes took place: the lifting of the limitation upon characters (prohibition of control code values), thus opening code points for allocation; and the synchronisation of the repertoire of the Basic Multilingual Plane with that of Unicode. Meanwhile, in the passage of time, the situation changed in the Unicode standard itself: 65,536 characters came to appear insufficient, and the standard from version 2.0 and onwards supports encoding of 1,112,064 code points from 17 planes by means of the

surrogate mechanism. For that reason, ISO/IEC 10646 was limited to contain as many characters as could be encoded by UTF-16 and no more, that is, a little over a million characters instead of over 679 million. The UCS-4 encoding of ISO/IEC 10646 was incorporated into the Unicode standard with the limitation to the UTF-16 range and under the name UTF-32, although it has almost no use outside programs' internal data.

Rob Pike Robert Pike (born 1956) is a Canadian programmer and author. He is best known for his work on the Go programming language while working at Google and the Plan 9 operating system while working at Bell Labs, where he was a member of the Unix t ...

and

Ken Thompson Kenneth Lane Thompson (born February 4, 1943) is an American pioneer of computer science. Thompson worked at Bell Labs for most of his career where he designed and implemented the original Unix operating system. He also invented the B (programmi ...

, the designers of the Plan 9 operating system, devised a new, fast and well-designed mixed-width encoding that was also backward-compatible with 7-bit

ASCII ASCII ( ), an acronym for American Standard Code for Information Interchange, is a character encoding standard for representing a particular set of 95 (English language focused) printable character, printable and 33 control character, control c ...

, which came to be called

UTF-8 UTF-8 is a character encoding standard used for electronic communication. Defined by the Unicode Standard, the name is derived from ''Unicode Transformation Format 8-bit''. Almost every webpage is transmitted as UTF-8. UTF-8 supports all 1,112,0 ...

, and is currently the most popular UCS encoding.

Differences from Unicode

ISO/IEC 10646 and Unicode have an identical repertoire and numbers—the same characters with the same numbers exist on both standards, although Unicode releases new versions and adds new characters more often. Unicode has rules and specifications outside the scope of ISO/IEC 10646. ISO/IEC 10646 is a simple character map, an extension of previous standards like

ISO/IEC 8859 ISO/IEC 8859 is a joint International Organization for Standardization, ISO and International Electrotechnical Commission, IEC series of standards for 8-bit character encodings. The series of standards consists of numbered parts, such as ISO/IEC ...

. In contrast, Unicode adds rules for

collation Collation is the assembly of written information into a standard order. Many systems of collation are based on numerical order or alphabetical order, or extensions and combinations thereof. Collation is a fundamental element of most office fi ...

, normalisation of forms, and the bidirectional algorithm for

right-to-left A writing system comprises a set of symbols, called a ''script'', as well as the rules by which the script represents a particular language. The earliest writing appeared during the late 4th millennium BC. Throughout history, each independen ...

scripts such as Arabic and Hebrew. For interoperability between platforms, especially if bidirectional scripts are used, it is not enough to support ISO/IEC 10646; Unicode must be implemented. To support these rules and algorithms, Unicode adds many

properties Property is the ownership of land, resources, improvements or other tangible objects, or intellectual property. Property may also refer to: Philosophy and science * Property (philosophy), in philosophy and logic, an abstraction characterizing an ...

to each character in the set such as properties determining a character's default bidirectional class and properties to determine how the character combines with other characters. If the character represents a numeric value such as the European number '8', or the vulgar fraction '¼', that numeric value is also added as a property of the character. Unicode intends these properties to support interoperable text handling with a mixture of languages. Some applications support ISO/IEC 10646 characters but do not fully support Unicode. One such application,

Xterm xterm is the standard terminal emulator for the X Window System. It allows users to run programs which require a command-line interface. If no particular program is specified, xterm runs the user's Unix shell, shell. An X display device, dis ...

, can properly display all ISO/IEC 10646 characters that have a one-to-one character-to-glyph mapping and a single directionality. It can handle some combining marks by simple overstriking methods, but cannot display Hebrew (bidirectional),

Devanagari Devanagari ( ; in script: , , ) is an Indic script used in the Indian subcontinent. It is a left-to-right abugida (a type of segmental Writing systems#Segmental systems: alphabets, writing system), based on the ancient ''Brāhmī script, Brā ...

(one character to many glyphs) or Arabic (both features). Most GUI applications use standard OS text drawing routines which handle such scripts, although the applications themselves still do not always handle them correctly.

Citing the Universal Coded Character Set

''ISO/IEC 10646'', a general, informal citation for the ISO/IEC 10646 family of standards, is acceptable in most prose. And even though it is a separate standard, the term ''Unicode'' is used just as often, informally, when discussing the UCS. However, any normative references to the UCS as a publication should cite the year of the edition in the form ''ISO/IEC 10646:'', for example: ''ISO/IEC 10646:2014''.

Relationship with Unicode

Since 1991, the

Unicode Consortium The Unicode Consortium (legally Unicode, Inc.) is a 501(c)(3) non-profit organization incorporated and based in Mountain View, California, U.S. Its primary purpose is to maintain and publish the Unicode Standard which was developed with the in ...

and the

/ IEC have developed ''

The Unicode Standard Unicode or ''The Unicode Standard'' or TUS is a character encoding standard maintained by the Unicode Consortium designed to support the use of text in all of the world's writing systems that can be digitized. Version 16.0 defines 154,998 ch ...

'' ("Unicode") and ISO/IEC 10646 in tandem. The repertoire, character names, and code points of Unicode Version 2.0 exactly match those of ISO/IEC 10646-1:1993 with its first seven published amendments. After Unicode 3.0 was published in February 2000, corresponding new and updated characters entered the UCS via ISO/IEC 10646-1:2000. In 2003, parts 1 and 2 of ISO/IEC 10646 were combined into a single part, which has since had a number of amendments adding characters to the standard in approximate synchrony with the Unicode standard. * ISO/IEC 10646-1:1993 = Unicode 1.1 * ISO/IEC 10646-1:1993 plus Amendments 5 to 7 = Unicode 2.0 * ISO/IEC 10646-1:1993 plus Amendments 5 to 7 = Unicode 2.1 excluding

Euro sign The euro sign () is the currency sign used for the euro, the official currency of the eurozone. The design was presented to the public by the European Commission on 12 December 1996. It consists of a stylized letter E (or epsilon), crossed by ...

and Object Replacement Character, which are included in Amendment 18 * ISO/IEC 10646-1:2000 = Unicode 3.0 * ISO/IEC 10646-1:2000 and ISO/IEC 10646-2:2001 = Unicode 3.1 * ISO/IEC 10646-1:2000 plus Amendment 1 and ISO/IEC 10646-2:2001 = Unicode 3.2 * ISO/IEC 10646:2003 = Unicode 4.0 * ISO/IEC 10646:2003 plus Amendment 1 = Unicode 4.1 * ISO/IEC 10646:2003 plus Amendments 1 to 2 = Unicode 5.0 excluding

letters GGA, JJA, DDDA and BBA, which are included in Amendment 3 * ISO/IEC 10646:2003 plus Amendments 1 to 4 = Unicode 5.1 * ISO/IEC 10646:2003 plus Amendments 1 to 6 = Unicode 5.2 * ISO/IEC 10646:2003 plus Amendments 1 to 8 = ISO/IEC 10646:2011 = Unicode 6.0 excluding Indian rupee sign * ISO/IEC 10646:2012 = Unicode 6.1 * ISO/IEC 10646:2012 = Unicode 6.2 excluding Turkish lira sign, which is included in Amendment 1 * ISO/IEC 10646:2012 = Unicode 6.3 excluding Turkish lira sign, which is included in Amendment 1, and five bidirectional control characters (Arabic Letter Mark, Left-To-Right Isolate, Right-To-Left Isolate, First Strong Isolate, Pop Directional Isolate), which are included in Amendment 2 * ISO/IEC 10646:2012 plus Amendments 1 and 2 = Unicode 7.0 excluding the

Ruble sign The ruble sign, , is the currency sign used for the Russian ruble, the official currency of Russia. Its form is a Cyrillic letter Р with an additional horizontal stroke. The design was approved on 11 December 2013 after a public poll that too ...

* ISO/IEC 10646:2014 plus Amendment 1 = Unicode 8.0 excluding the Lari sign, nine CJK unified ideographs, and 41 emoji characters * ISO/IEC 10646:2014 plus Amendments 1 and 2 = Unicode 9.0 excluding Adlam, Newa, Japanese TV symbols, and 74 emoji and symbols * ISO/IEC 10646:2017 = Unicode 10.0 excluding 285

Hentaigana In the Japanese writing system, are variant forms of hiragana. Description In contrast to modern Japanese, originally hiragana had several forms for a single sound. For example, while the hiragana reading "ha" has only one form in modern ...

characters, 3 Zanabazar Square characters, and 56 emoji symbols * ISO/IEC 10646:2017 plus Amendment 1 = Unicode 11.0 excluding 46 Mtavruli Georgian capital letters, 5 CJK unified ideographs, and 66 emoji characters * ISO/IEC 10646:2017 plus Amendments 1 and 2 = Unicode 12.0 excluding 62 additional characters * ISO/IEC 10646:2020 = Unicode 13.0 * ISO/IEC 10646:2020 plus Amendments 1 = Unicode 15.0 * ISO/IEC 10646:2020 plus Amendments 1 and 2 = Unicode 16.0

References

External links

Publicly available standards
(ISO) – includes a copy of ISO/IEC 10646:2020/Amd. 1:2023(E)
ISO/IEC JTC1/SC2/WG2
the

working group A working group is a group of experts working together to achieve specified goals. Such groups are domain-specific and focus on discussion or activity around a specific subject area. The term can sometimes refer to an interdisciplinary collab ...