Duplicate characters in Unicode

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 U+00B5 µMICRO SIGN versus U+03BC μGREEK SMALL LETTER MU.

This should be clearly distinguished from Unicode characters that are rendered as identical glyphs or near-identical glyphs (homoglyphs), 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 ʘ, 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 glyphs. 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 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 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 acute accent in its various meanings is considered one and the same combining diacritic character U+0301 ́COMBINING ACUTE ACCENT, and so the accented letter é is the same character in French and Hungarian. There is a separate "combining diacritic acute tone mark" at U+0341 ́COMBINING ACUTE TONE MARK 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 U+0384 ΄GREEK TONOS, and for the Armenian alphabet at U+055B ՛ARMENIAN EMPHASIS MARK. 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). 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).

Compatibility issues

CJK fullwidth forms

In traditional Chinese character 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.

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 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" ℒ vs. "blackletter L" 𝔏 vs. "boldface blackletter L" 𝕷) as distinctive mathematical symbols. It is intended for use only in mathematical or technical notation, not use in non-technical text.^[1]

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" (U+00B5 µMICRO 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 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, 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 uppercase and lowercase numerals, as well as pre-combined glyphs for numbers up to 12 (Ⅻ for XII), 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 Ⅹ with Ⅰ to produce Roman numeral 11 (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, ↇ, ↈ, and Ↄ; 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, U+0647 هARABIC LETTER HEH has its contextual forms encoded at these 4 code points:

U+FEE9 ﻩARABIC LETTER HEH ISOLATED FORM
U+FEEA ﻪARABIC LETTER HEH FINAL FORM
U+FEEB ﻫARABIC LETTER HEH INITIAL FORM
U+FEEC ﻬARABIC LETTER HEH MEDIAL FORM

The contextual-form characters are not recommended for general use. There are also compatibility Arabic ligatures encoded such as U+FDF2 ﷲARABIC LIGATURE ALLAH ISOLATED FORM and U+FDFD ﷽ARABIC LIGATURE BISMILLAH AR-RAHMAN AR-RAHEEM.

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 U+FB4F ﭏHEBREW LIGATURE ALEF LAMED. The wide variants are listed below:

U+FB21 ﬡHEBREW LETTER WIDE ALEF
U+FB22 ﬢHEBREW LETTER WIDE DALET
U+FB23 ﬣHEBREW LETTER WIDE HE
U+FB24 ﬤHEBREW LETTER WIDE KAF
U+FB25 ﬥHEBREW LETTER WIDE LAMED
U+FB26 ﬦHEBREW LETTER WIDE FINAL MEM
U+FB27 ﬧHEBREW LETTER WIDE RESH
U+FB28 ﬨHEBREW LETTER WIDE TAV

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 U+05E2 עHEBREW LETTER AYIN at U+FB20 ﬠHEBREW LETTER ALTERNATIVE AYIN.

List

U+1F549 🕉OM SYMBOL: U+0950 ॐDEVANAGARI OM
U+212B ÅANGSTROM SIGN: U+00C5 ÅLATIN CAPITAL LETTER A WITH RING ABOVE
U+00B5 µMICRO SIGN: U+03BC μGREEK SMALL LETTER MU
U+037E ;GREEK QUESTION MARK: U+003B ;SEMICOLON
U+212A KKELVIN SIGN: U+004B KLATIN CAPITAL LETTER K
U+2024 ․ONE DOT LEADER: U+002E .FULL STOP
U+2126 ΩOHM SIGN: U+03A9 ΩGREEK CAPITAL LETTER OMEGA
U+2236 ∶RATIO: U+003A :COLON
U+0387 ·GREEK ANO TELEIA: U+00B7 ·MIDDLE DOT
U+2A75 ⩵TWO CONSECUTIVE EQUALS SIGNS: U+003D =EQUALS SIGN, U+003D =EQUALS SIGN
U+2A76 ⩶THREE CONSECUTIVE EQUALS SIGNS: U+003D =EQUALS SIGN, U+003D =EQUALS SIGN, U+003D =EQUALS SIGN
U+27EAF 𧺯CJK UNIFIED IDEOGRAPH-27EAF: U+FA23 﨣CJK COMPATIBILITY IDEOGRAPH-FA23
U+2135 ℵALEF SYMBOL: U+05D0 אHEBREW LETTER ALEF
U+2136 ℶBET SYMBOL: U+05D1 בHEBREW LETTER BET
U+2137 ℷGIMEL SYMBOL: U+05D2 גHEBREW LETTER GIMEL
U+2138 ℸDALET SYMBOL: U+05D3 דHEBREW LETTER DALET
U+2254 ≔COLON EQUALS: U+003A :COLON, U+003D =EQUALS SIGN
U+2255 ≕EQUALS COLON: U+003D =EQUALS SIGN, U+003A :COLON
U+2A74 ⩴DOUBLE COLON EQUAL: U+003A :COLON, U+003A :COLON, U+003D =EQUALS SIGN
U+0340 ◌̀COMBINING GRAVE TONE MARK: U+0300 ◌̀COMBINING GRAVE ACCENT
U+0341 ◌́COMBINING ACUTE TONE MARK: U+0301 ◌́COMBINING ACUTE ACCENT
U+0344 ◌̈́COMBINING GREEK DIALYTIKA TONOS: U+0308 ◌̈COMBINING DIAERESIS, U+0301 ◌́COMBINING ACUTE ACCENT
U+222C ∬DOUBLE INTEGRAL: U+222B ∫INTEGRAL, U+222B ∫INTEGRAL
U+222D ∭TRIPLE INTEGRAL: U+222B ∫INTEGRAL, U+222B ∫INTEGRAL, U+222B ∫INTEGRAL
U+2A0C ⨌QUADRUPLE INTEGRAL OPERATOR: U+222B ∫INTEGRAL, U+222B ∫INTEGRAL, U+222B ∫INTEGRAL, U+222B ∫INTEGRAL
U+03D0 ϐGREEK BETA SYMBOL: U+03B2 βGREEK SMALL LETTER BETA
U+03F4 ϴGREEK CAPITAL THETA SYMBOL: U+0398 ΘGREEK CAPITAL LETTER THETA
U+03D1 ϑGREEK THETA SYMBOL: U+03B8 θGREEK SMALL LETTER THETA
U+03D6 ϖGREEK PI SYMBOL: U+03C0 πGREEK SMALL LETTER PI
U+03F1 ϱGREEK RHO SYMBOL: U+03C1 ρGREEK SMALL LETTER RHO
U+03D2 ϒGREEK UPSILON WITH HOOK SYMBOL: U+03A5 ΥGREEK CAPITAL LETTER UPSILON
U+03D3 ϓGREEK UPSILON WITH ACUTE AND HOOK SYMBOL: U+038E ΎGREEK CAPITAL LETTER UPSILON WITH TONOS
U+03D4 ϔGREEK UPSILON WITH DIAERESIS AND HOOK SYMBOL: U+03AB ΫGREEK CAPITAL LETTER UPSILON WITH DIALYTIKA
U+03D5 ϕGREEK PHI SYMBOL: U+03C6 φGREEK SMALL LETTER PHI
U+0374 ʹGREEK NUMERAL SIGN: U+02B9 ʹMODIFIER LETTER PRIME
U+03F0 ϰGREEK KAPPA SYMBOL: U+03BA κGREEK SMALL LETTER KAPPA
U+03F9 ϹGREEK CAPITAL LUNATE SIGMA SYMBOL: U+03A3 ΣGREEK CAPITAL LETTER SIGMA
U+03F2 ϲGREEK LUNATE SIGMA SYMBOL: U+03C3 σGREEK SMALL LETTER SIGMA
U+017F ſLATIN SMALL LETTER LONG S: U+0073 sLATIN SMALL LETTER S
U+03F5 ϵGREEK LUNATE EPSILON SYMBOL: U+03B5 εGREEK SMALL LETTER EPSILON
U+210F ℏPLANCK CONSTANT OVER TWO PI: U+0127 ħLATIN SMALL LETTER H WITH STROKE
U+2107 ℇEULER CONSTANT: U+0190 ƐLATIN CAPITAL LETTER OPEN E
U+2103 ℃DEGREE CELSIUS: U+00B0 °DEGREE SIGN, U+0043 CLATIN CAPITAL LETTER C
U+2109 ℉DEGREE FAHRENHEIT: U+00B0 °DEGREE SIGN, U+0046 FLATIN CAPITAL LETTER F
U+00BA ºMASCULINE ORDINAL INDICATOR: U+006F oLATIN SMALL LETTER O
U+00AA ªFEMININE ORDINAL INDICATOR: U+0061 aLATIN SMALL LETTER A
U+2139 ℹINFORMATION SOURCE: U+0069 iLATIN SMALL LETTER I
U+FB20 ﬠHEBREW LETTER ALTERNATIVE AYIN: U+05E2 עHEBREW LETTER AYIN
U+FB21 ﬡHEBREW LETTER WIDE ALEF: U+05D0 אHEBREW LETTER ALEF
U+FB22 ﬢHEBREW LETTER WIDE DALET: U+05D3 דHEBREW LETTER DALET
U+FB23 ﬣHEBREW LETTER WIDE HE: U+05D4 הHEBREW LETTER HE
U+FB24 ﬤHEBREW LETTER WIDE KAF: U+05DB כHEBREW LETTER KAF
U+FB25 ﬥHEBREW LETTER WIDE LAMED: U+05DC לHEBREW LETTER LAMED
U+FB26 ﬦHEBREW LETTER WIDE FINAL MEM: U+05DD םHEBREW LETTER FINAL MEM
U+FB27 ﬧHEBREW LETTER WIDE RESH: U+05E8 רHEBREW LETTER RESH
U+FB28 ﬨHEBREW LETTER WIDE TAV: U+05EA תHEBREW LETTER TAV
U+FB29 ﬩HEBREW LETTER ALTERNATIVE PLUS SIGN: U+002B +PLUS SIGN
U+0343 ◌̓COMBINING GREEK KORONIS: U+0313 ◌̓COMBINING COMMA ABOVE
U+1FFD ´GREEK OXIA: U+00B4 ´ACUTE ACCENT
U+0384 ΄GREEK TONOS: U+00B4 ´ACUTE ACCENT
U+1FEF `GREEK VARIA: U+0060 `GRAVE ACCENT

References

^"UTR #25: Unicode and Mathematics". unicode.org. Retrieved 2024-03-04.

[1] "UTR #25: Unicode and Mathematics". unicode.org. Retrieved 2024-03-04.

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