Duplicate characters in Unicode
Encyclopedia
Unicode
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 cases such as the "micro sign" µ vs. the Greek μ.
This should be clearly distinguished from Unicode characters that are rendered as identical glyphs or near-identical glyphs (homoglyph
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 astronomical symbol for "Sun" ☉ vs. "circled dot operator" ⊙ vs. the Gothic letter 𐍈 vs. the IPA symbol for a bilabial click ʘ).
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- sign" µ separate from Greek μ, but not a "Mega sign" separate from Latin M was a pragmatic decision by Unicode consortium for historical reasons (compatibility with Latin-1 which included 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
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 Latin alphabet
, the acute accent in its various meanings is considered one and the same combining diacritic character (U+0301). There is a separate "combining diacritic acute tone mark" at U+0341 for the romanization of tone languages, one important difference between the two 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 U+0384, and for the Armenian alphabet at U+055B (but the Cyrillic alphabet, which also uses the acute accent, does not have a "Cyrillic acute" encoded separately and U+301 should be used for Cyrillic as well as Latin, see Cyrillic characters in Unicode
). 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 /juː/, /ʊ/, /ʌ/ etc., French /y/, German /uː/, /u/, etc., not to mention various uses of U as a symbol
).
encodings characters usually took either a single byte
(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
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.
Several variants of the entire Greek and Latin alphabets specifically for use as mathematical symbols are encoded in the Mathematical alphanumeric symbols
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. It is intended for use only in mathematical or technical notation, not use in non-technical text.
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" (U+00B5, µ) 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 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 archaic Ϙ ϙ.
Greek letters assigned separate "symbol" codepoints include the beta, epsilon, theta, pi, rho, Ypsilon and phi symbols ϐ, ϵ, ϑ, ϖ, ϱ, ϒ, ϕ contrasting with β, ε, θ, π, ρ, Υ, φ, in the Letterlike Symbols
range the Ohm symbol Ω contrasting with Ω, and the Mathematical Operators range the n-ary product symbol ∏ (U+220F) contrasting with Π (U+03A0) and the sum operator ∑ (and "Latin esh" Ʃ U+01A9) contrasting with Σ.
, as part of the Number Forms range from U+2160 to U+2183. For example, Roman 1988 (MCMLXXXVIII) could alternatively be written as ⅯⅭⅯⅬⅩⅩⅩⅧ. This range includes both upper- and lowercase numerals, as well as pre-combined glyphs for numbers up to 12 (Ⅻ for XII), mainly intended for clock faces. Similarly precombined ligatures for 5,000 and 10,000 exist.
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 Ⅹ with Ⅰ to mean roman numeral eleven (ⅩⅠ), so U+216A (Ⅺ) is canonically equivalent to ⅩⅠ. 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: Ⅰ, Ⅴ, Ⅹ, Ⅼ, Ⅽ, Ⅾ, Ⅿ, ⅰ, ⅴ, ⅹ, ⅼ, ⅽ, ⅾ, ⅿ, ↀ, ↁ, ↂ, Ↄ; all other roman numerals can be composed from these.
Unicode
Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...
has a certain amount of duplication of characters
Character (computing)
In computer and machine-based telecommunications terminology, a character is a unit of information that roughly corresponds to a grapheme, grapheme-like unit, or symbol, such as in an alphabet or syllabary in the written form of a natural language....
. 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 cases such as the "micro sign" µ vs. the Greek μ.
This should be clearly distinguished from Unicode characters that are rendered as identical glyphs or near-identical glyphs (homoglyph
Homoglyph
In typography, a homoglyph is one of two or more characters, or glyphs, with shapes that either appear identical or cannot be differentiated by quick visual inspection. This designation is also applied to sequences of characters sharing these properties....
s), either because they are historically cognate (such as Greek Η vs. Latin H
H
H .) is the eighth letter in the basic modern Latin alphabet.-History:The Semitic letter ⟨ח⟩ most likely represented the voiceless pharyngeal fricative . The form of the letter probably stood for a fence or posts....
) or because of coincidental similarity (such as Greek Ρ vs. Latin P
P
P is the sixteenth letter of the basic modern Latin alphabet.-Usage:In English and most other European languages, P is a voiceless bilabial plosive. Both initial and final Ps can be combined with many other discrete consonants in English words...
, or Greek Η vs. Cyrillic Н, or the astronomical symbol for "Sun" ☉ vs. "circled dot operator" ⊙ vs. the Gothic letter 𐍈 vs. the IPA symbol for a bilabial click ʘ).
Duplicate vs. derived character
Unicode aims at encoding graphemes, not individual "meanings" ("semantics") of graphemes, and not glyphGlyph
A glyph is an element of writing: an individual mark on a written medium that contributes to the meaning of what is written. A glyph is made up of one or more graphemes....
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- sign" µ separate from Greek μ, but not a "Mega sign" separate from Latin M was a pragmatic decision by Unicode consortium for historical reasons (compatibility with Latin-1 which included 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
Acute accent
The acute accent is a diacritic used in many modern written languages with alphabets based on the Latin, Cyrillic, and Greek scripts.-Apex:An early precursor of the acute accent was the apex, used in Latin inscriptions to mark long vowels.-Greek:...
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 Latin alphabet
Latin alphabet
The Latin alphabet, also called the Roman alphabet, is the most recognized alphabet used in the world today. It evolved from a western variety of the Greek alphabet called the Cumaean alphabet, which was adopted and modified by the Etruscans who ruled early Rome...
, the acute accent in its various meanings is considered one and the same combining diacritic character (U+0301). There is a separate "combining diacritic acute tone mark" at U+0341 for the romanization of tone languages, one important difference between the two 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 U+0384, and for the Armenian alphabet at U+055B (but the Cyrillic alphabet, which also uses the acute accent, does not have a "Cyrillic acute" encoded separately and U+301 should be used for Cyrillic as well as Latin, see Cyrillic characters in Unicode
Cyrillic characters in Unicode
The Cyrillic script is encoded in four blocks in Unicode, all in BMP:* Cyrillic: , 256 characters* Cyrillic Supplement: , 48 characters* Cyrillic Extended-A: , 32 characters* Cyrillic Extended-B: , 96 characters...
). The point that the same grapheme can have many "meanings" is even more obvious considering e.g. the letter U
U
U is the twenty-first letter and a vowel in the basic modern Latin alphabet.-History:The letter U ultimately comes from the Semitic letter Waw by way of the letter Y. See the letter Y for details....
, which has entirely different phonemic referents in the various languages that use it in their orthographies (English /juː/, /ʊ/, /ʌ/ etc., French /y/, German /uː/, /u/, etc., not to mention various uses of U as a symbol
U (disambiguation)
U is the twenty-first letter of the Latin alphabet.U may also refer to:-Mathematics:* \cup, union * U-set, a set of uniqueness* U, the unitary group-Chemistry:* Uranium, a metallic chemical element...
).
CJK fullwidth forms
In traditional CJKCJK
CJK is a collective term for Chinese, Japanese, and Korean, which is used in the field of software and communications internationalization.The term CJKV means CJK plus Vietnamese, which constitute the main East Asian languages.- Characteristics :...
encodings characters usually took either a single byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
(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
Latin alphabet
The Latin alphabet, also called the Roman alphabet, is the most recognized alphabet used in the world today. It evolved from a western variety of the Greek alphabet called the Cumaean alphabet, which was adopted and modified by the Etruscans who ruled early Rome...
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 a mathematical constant 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
Mathematical Alphanumeric Symbols is a Unicode block of Latin and Greek letters and decimal digits that enable mathematicians to denote different notions with different letter styles .Unicode now includes many such symbols Mathematical Alphanumeric Symbols is a Unicode block of Latin and Greek...
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
L
Ł or ł, described in English as L with stroke, is a letter of the Polish, Kashubian, Sorbian, Łacinka , Łatynka , Wilamowicean, Navajo, Dene Suline, Inupiaq, Zuni, Hupa, and Dogrib alphabets, several proposed alphabets for the Venetian language, and the ISO 11940 romanization of the Thai alphabet...
vs. "script L" vs. "blackletter L" vs. "boldface blackletter L" ) as distinctive mathematical symbols. It is intended for use only in mathematical or technical notation, not use in non-technical text.
Greek
Many Greek lettersGreek alphabet
The Greek alphabet is the script that has been used to write the Greek language since at least 730 BC . The alphabet in its classical and modern form consists of 24 letters ordered in sequence from alpha to omega...
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" (U+00B5, µ) 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 Ϲ ϲ contrasting with Σ σ, final sigma ς (strictly speaking a contextual glyph variant) contrasting with σ, The Qoppa
Qoppa
Koppa or Qoppa is a letter that was used in early forms of the Greek alphabet, derived from Phoenician qoph. It was originally used to denote the /k/ sound, but dropped out of use as an alphabetic character in favour of Kappa . It has remained in use as a numeral symbol in the system of Greek...
numeral symbol Ϟ ϟ contrasting with archaic Ϙ ϙ.
Greek letters assigned separate "symbol" codepoints include the beta, epsilon, theta, pi, rho, Ypsilon and phi symbols ϐ, ϵ, ϑ, ϖ, ϱ, ϒ, ϕ contrasting with β, ε, θ, π, ρ, Υ, φ, in the Letterlike Symbols
Letterlike Symbols
Letterlike Symbols are graphemes which are constructed mainly from the glyphs of one or more letters.In Unicode, Letterlike Symbols are placed in the block U+2100–214F, as in the following table.-See also:*Mapping of Unicode characters...
range the Ohm symbol Ω contrasting with Ω, and the Mathematical Operators range the n-ary product symbol ∏ (U+220F) contrasting with Π (U+03A0) and the sum operator ∑ (and "Latin esh" Ʃ U+01A9) contrasting with Σ.
Roman numerals
Unicode has a number of characters specifically designated as Roman numeralsRoman numerals
The numeral system of ancient Rome, or Roman numerals, uses combinations of letters from the Latin alphabet to signify values. The numbers 1 to 10 can be expressed in Roman numerals as:...
, as part of the Number Forms range from U+2160 to U+2183. For example, Roman 1988 (MCMLXXXVIII) could alternatively be written as ⅯⅭⅯⅬⅩⅩⅩⅧ. This range includes both upper- and lowercase numerals, as well as pre-combined glyphs for numbers up to 12 (Ⅻ for XII), mainly intended for clock faces. Similarly precombined ligatures for 5,000 and 10,000 exist.
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 Ⅹ with Ⅰ to mean roman numeral eleven (ⅩⅠ), so U+216A (Ⅺ) is canonically equivalent to ⅩⅠ. 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
Unicode compatibility characters
In discussing Unicode and the UCS, many often refer to compatibility characters. Compatibility characters are graphical characters that are discouraged by the Unicode Consortium...
). 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: Ⅰ, Ⅴ, Ⅹ, Ⅼ, Ⅽ, Ⅾ, Ⅿ, ⅰ, ⅴ, ⅹ, ⅼ, ⅽ, ⅾ, ⅿ, ↀ, ↁ, ↂ, Ↄ; all other roman numerals can be composed from these.