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 ...
has a certain amount of duplication of
characters. These are pairs of single Unicode code points that are
canonically equivalent. The reason for this are compatibility issues with legacy systems.
Unless two characters are canonically equivalent, they are not "duplicate" in the narrow sense. There is, however, room for disagreement on whether two Unicode characters really encode the same
grapheme
In linguistics, a grapheme is the smallest functional unit of a writing system.
The word ''grapheme'' is derived from Ancient Greek ('write'), and the suffix ''-eme'' by analogy with ''phoneme'' and other emic units. The study of graphemes ...
in cases such as the versus .
This should be clearly distinguished from Unicode characters that are rendered as identical glyphs or near-identical glyphs (
homoglyph
In orthography and typography, a homoglyph is one of two or more graphemes, character (computing), characters, or glyphs with shapes that appear identical or very similar but may have differing meaning. The designation is also applied to sequence ...
s), either because they are historically cognate (such as Greek
Η vs. Latin
H) or because of coincidental similarity (such as Greek
Ρ vs. Latin
P, or Greek Η vs. Cyrillic
Н, or the following homoglyph septuplet: astronomical symbol for "Sun"
☉, "circled dot operator"
⊙, the Gothic letter
𐍈, the IPA symbol for a bilabial click , the
Osage letter 𐓃, the
Tifinagh letter ⵙ, and the archaic Cyrillic letter
Ꙩ).
Duplicate vs. derived character
Unicode aims at encoding graphemes, not individual "meanings" ("semantics") of graphemes, and not
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 ...
s.
It is a matter of case-by-case judgement whether such characters should receive separate encoding when used in technical contexts, e.g. Greek letters used as mathematical symbols: thus, the choice to have a "
micro-
''Micro'' (Greek letter μ, Mu (letter), mu, non-Italic type, italic) is a metric prefix, unit prefix in the metric system denoting a factor of one millionth (10−6). It comes from the Ancient Greek, Greek word (), meaning "small".
It is the ...
sign" µ separate from Greek μ, but not a "
Mega sign" separate from Latin M, was a pragmatic decision by The Unicode Consortium for historical reasons (namely, compatibility with
Latin-1, which includes a micro sign). Technically µ and μ are not duplicate characters in that the consortium viewed these symbols as distinct characters (while it regarded M for "Mega" and Latin M as one and the same character).
Note that merely having different "meanings" is not sufficient grounds to split a grapheme into several characters. Thus, the
acute accent
The acute accent (), ,
is a diacritic used in many modern written languages with alphabets based on the Latin alphabet, Latin, Cyrillic script, Cyrillic, and Greek alphabet, Greek scripts. For the most commonly encountered uses of the accen ...
may represent word accent in Welsh or Swedish, it may express vowel quality in French, and it may express vowel length in Hungarian, Icelandic or Irish. Since all these languages are written in the same
script, namely
Latin script
The Latin script, also known as the Roman script, is a writing system based on the letters of the classical Latin alphabet, derived from a form of the Greek alphabet which was in use in the ancient Greek city of Cumae in Magna Graecia. The Gree ...
, the acute accent in its various meanings is considered one and the same combining diacritic character , and so the accented letter
é is the same character in French and Hungarian. There is a separate "combining diacritic acute tone mark" at for the romanization of tone languages, one important difference from the acute accent being that in a language like French, the acute accent can replace the dot over the lowercase i, whereas in a language like Vietnamese, the acute tone mark is added above the dot. Diacritic signs for alphabets considered independent may be encoded separately, such as the acute ("tonos") for the Greek alphabet at , and for the Armenian alphabet at . Some Cyrillic-based alphabets (such as
Russian) also use the acute accent, but there is no "Cyrillic acute" encoded separately and U+0301 should be used for Cyrillic as well as Latin (see
Cyrillic characters in Unicode
As of Unicode version , Cyrillic script is encoded across several blocks:
* CyrillicU+0400–U+04FF 256 characters
* Cyrillic SupplementU+0500–U+052F 48 characters
* Cyrillic Extended-AU+2DE0–U+2DFF 32 characters
* Cyrillic Extended-BU+A64 ...
). The point that the same grapheme can have many "meanings" is even more obvious considering e.g. the letter
U, which has entirely different phonemic referents in the various languages that use it in their orthographies (English etc., French , German , etc., not to mention various uses of
U as a symbol).
Compatibility issues
CJK fullwidth forms
In traditional
Chinese character encodings, characters usually took either a single
byte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...
(known as halfwidth) or two bytes (known as fullwidth). Characters that took a single byte were generally displayed at half the width of those that took two bytes. Some characters such as the
Latin alphabet
The Latin alphabet, also known as the Roman alphabet, is the collection of letters originally used by the Ancient Rome, ancient Romans to write the Latin language. Largely unaltered except several letters splitting—i.e. from , and from � ...
were available in both halfwidth and fullwidth versions. As the halfwidth versions were more commonly used, they were generally the ones mapped to the standard code points for those characters. Therefore a separate section was needed for the fullwidth forms to preserve the distinction.
Letterlike symbols
In some cases, specific graphemes have acquired a specialized symbolic or technical meaning separate from their original function. A prominent example is the Greek letter
π which is widely recognized as the symbol for the mathematical constant of a circle's circumference divided by its diameter even by people not literate in Greek.
Several variants of the entire Greek and Latin alphabets specifically for use as mathematical symbols are encoded in the
Mathematical Alphanumeric Symbols
Mathematical Alphanumeric Symbols is a Unicode block comprising styled forms of Latin alphabet, Latin and Greek alphabet, Greek letters and decimal numerical digit, digits that enable mathematicians to denote different notions with different l ...
range. This range disambiguates characters that would usually be considered font variants but are encoded separately because of widespread use of font variants e.g.
L vs. "script L" vs. "blackletter L" vs. "boldface blackletter L" ) as distinctive
mathematical symbols
A mathematical symbol is a figure or a combination of figures that is used to represent a mathematical object, an action on mathematical objects, a relation between mathematical objects, or for structuring the other symbols that occur in a mathemat ...
. It is intended for use only in mathematical or technical notation, not use in non-technical text.
Greek
Many
Greek letters are used as
technical symbols. All of the Greek letters are encoded in the Greek section of Unicode but many are encoded a second time under the name of the technical symbol they represent. The "
micro sign" () is obviously inherited from
ISO 8859-1, but the origin of the others is less clear.
Other Greek glyph variants encoded as separate characters include the
lunate sigma
Sigma ( ; uppercase Σ, lowercase σ, lowercase in word-final position ς; ) is the eighteenth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 200. In general mathematics, uppercase Σ is used as an operator ...
Ϲ ϲ contrasting with Σ σ, final sigma ς (strictly speaking a contextual glyph variant) contrasting with σ, The
Qoppa numeral symbol Ϟ ϟ contrasting with the archaic Ϙ ϙ.
Greek letters assigned separate "symbol" codepoints include the
Letterlike Symbols ϐ,
ϵ,
ϑ,
ϖ,
ϱ,
ϒ, and
ϕ (contrasting with β, ε, θ, π, ρ, Υ, φ); the Ohm symbol
Ω (contrasting with Ω); and the
mathematical operators for the product
∏ and sum
∑ (contrasting with
Π and
Σ).
Roman numerals
Unicode has a number of characters specifically designated as
Roman numerals
Roman numerals are a numeral system that originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. Numbers are written with combinations of letters from the Latin alphabet, eac ...
, as part of the
Number Forms range from U+2160 to U+2183. For example, Roman 1988 () could alternatively be written as . This range includes both uppercase and lowercase numerals, as well as pre-combined glyphs for numbers up to 12 ( for ), mainly intended for clock faces.
The pre-combined glyphs should only be used to represent the individual numbers where the use of individual glyphs is not wanted, and not to replace compounded numbers. For example, one can combine with to produce Roman numeral 11 (), so U+216A () is canonically equivalent to . Such characters are also referred to as composite compatibility characters or decomposable compatibility characters. Such characters would not normally have been included within the Unicode standard except for compatibility with other existing encodings (see
Unicode compatibility characters). The goal was to accommodate simple translation from existing encodings into Unicode. This makes translations in the opposite direction complicated because multiple Unicode characters may map to a single character in another encoding. Without the compatibility concerns the only characters necessary would be: , , , , , , , , , , , , , , , , , , , and ; all other Roman numerals can be composed from these characters.
Arabic presentation forms
Unicode has encoded compatibility characters for contextual Arabic letter forms where its contextual forms are encoded as separate code points (isolated, final, initial, and medial). For example, has its contextual forms encoded at these 4 code points:
*
*
*
*
The contextual-form characters are not recommended for general use. There are also compatibility Arabic ligatures encoded such as and .
Hebrew presentation forms
Hebrew presentation forms include ligatures, several precomposed characters and wide variants of Hebrew letters. The aleph-lamed ligature is encoded as a separate character at .
The wide variants are listed below:
*
*
*
*
*
*
*
*
These characters are variants of ordinary Hebrew letters encoded for
justification of texts written in Hebrew, such as the Torah. Unicode also encodes a stylistic variant of at .
List
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See also
*
IDN homograph attack
The internationalized domain name (IDN) homograph attack (sometimes written as homoglyph attack) is a method used by malicious parties to deceive computer users about what remote system they are communicating with, by exploiting the fact that man ...
*
Unicode equivalence
*
Homoglyph
In orthography and typography, a homoglyph is one of two or more graphemes, character (computing), characters, or glyphs with shapes that appear identical or very similar but may have differing meaning. The designation is also applied to sequence ...
*
ASCII art
References
{{Unicode navigation
Unicode