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+00B5µ MICRO SIGN versus U+03BCμ GREEK 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+0301́ 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+0341́ 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.

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+00B5µ 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 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, 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 Greek letters Pi Π and Sigma 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+0647ه ARABIC LETTER HEH has its contextual forms encoded at these 4 code points:

U+FEE9ﻩ ARABIC LETTER HEH ISOLATED FORM
U+FEEAﻪ ARABIC LETTER HEH FINAL FORM
U+FEEBﻫ ARABIC LETTER HEH INITIAL FORM
U+FEECﻬ ARABIC LETTER HEH MEDIAL FORM

The contextual-form characters are not recommended for general use. There are also compatibility Arabic ligatures encoded such as U+FDF2ﷲ ARABIC 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+FB4Fﭏ HEBREW LIGATURE ALEF LAMED. The wide variants are listed below:

U+FB21ﬡ HEBREW LETTER WIDE ALEF
U+FB22ﬢ HEBREW LETTER WIDE DALET
U+FB23ﬣ HEBREW LETTER WIDE HE
U+FB24ﬤ HEBREW LETTER WIDE KAF
U+FB25ﬥ HEBREW LETTER WIDE LAMED
U+FB26ﬦ HEBREW LETTER WIDE FINAL MEM
U+FB27ﬧ HEBREW LETTER WIDE RESH
U+FB28ﬨ HEBREW 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+05E2ע HEBREW LETTER AYIN at U+FB20ﬠ HEBREW LETTER ALTERNATIVE AYIN.

List

One-to-one mappings

U+00AAª FEMININE ORDINAL INDICATOR: U+0061a LATIN SMALL LETTER A
U+00B5µ MICRO SIGN: U+03BCμ GREEK SMALL LETTER MU
U+00BAº MASCULINE ORDINAL INDICATOR: U+006Fo LATIN SMALL LETTER O
U+017Fſ LATIN SMALL LETTER LONG S: U+0073s LATIN SMALL LETTER S
U+0340◌̀ COMBINING GRAVE TONE MARK: U+0300◌̀ COMBINING GRAVE ACCENT
U+0341◌́ COMBINING ACUTE TONE MARK: U+0301◌́ COMBINING ACUTE ACCENT
U+0343◌̓ COMBINING GREEK KORONIS: U+0313◌̓ COMBINING COMMA ABOVE
U+0374ʹ GREEK NUMERAL SIGN: U+02B9ʹ MODIFIER LETTER PRIME
U+037E; GREEK QUESTION MARK: U+003B; SEMICOLON
U+0384΄ GREEK TONOS: U+00B4´ ACUTE ACCENT
U+0387· GREEK ANO TELEIA: U+00B7· MIDDLE DOT
U+03D0ϐ GREEK BETA SYMBOL: U+03B2β GREEK SMALL LETTER BETA
U+03D1ϑ GREEK THETA SYMBOL: U+03B8θ GREEK SMALL LETTER THETA
U+03D5ϕ GREEK PHI SYMBOL: U+03C6φ GREEK SMALL LETTER PHI
U+03D6ϖ GREEK PI SYMBOL: U+03C0π GREEK SMALL LETTER PI
U+03F0ϰ GREEK KAPPA SYMBOL: U+03BAκ GREEK SMALL LETTER KAPPA
U+03F1ϱ GREEK RHO SYMBOL: U+03C1ρ GREEK SMALL LETTER RHO
U+03F2ϲ GREEK LUNATE SIGMA SYMBOL: U+03C3σ GREEK SMALL LETTER SIGMA
U+03F4ϴ GREEK CAPITAL THETA SYMBOL: U+0398Θ GREEK CAPITAL LETTER THETA
U+03F5ϵ GREEK LUNATE EPSILON SYMBOL: U+03B5ε GREEK SMALL LETTER EPSILON
U+03F9Ϲ GREEK CAPITAL LUNATE SIGMA SYMBOL: U+03A3Σ GREEK CAPITAL LETTER SIGMA
U+1FEF` GREEK VARIA: U+0060` GRAVE ACCENT
U+1FFD´ GREEK OXIA: U+00B4´ ACUTE ACCENT
U+2024․ ONE DOT LEADER: U+002E. FULL STOP
U+2107ℇ EULER CONSTANT: U+0190Ɛ LATIN CAPITAL LETTER OPEN E
U+210Eℎ PLANCK CONSTANT: U+0068h LATIN SMALL LETTER H
U+210Fℏ PLANCK CONSTANT OVER TWO PI: U+0127ħ LATIN SMALL LETTER H WITH STROKE
U+2126Ω OHM SIGN: U+03A9Ω GREEK CAPITAL LETTER OMEGA
U+212AK KELVIN SIGN: U+004BK LATIN CAPITAL LETTER K
U+212BÅ ANGSTROM SIGN: U+00C5Å LATIN CAPITAL LETTER A WITH RING ABOVE
U+2135ℵ ALEF SYMBOL: U+05D0א HEBREW LETTER ALEF
U+2136ℶ BET SYMBOL: U+05D1ב HEBREW LETTER BET
U+2137ℷ GIMEL SYMBOL: U+05D2ג HEBREW LETTER GIMEL
U+2138ℸ DALET SYMBOL: U+05D3ד HEBREW LETTER DALET
U+2139ℹ INFORMATION SOURCE: U+0069i LATIN SMALL LETTER I
U+2236∶ RATIO: U+003A: COLON
U+FB20ﬠ HEBREW LETTER ALTERNATIVE AYIN: U+05E2ע HEBREW LETTER AYIN
U+FB21ﬡ HEBREW LETTER WIDE ALEF: U+05D0א HEBREW LETTER ALEF
U+FB22ﬢ HEBREW LETTER WIDE DALET: U+05D3ד HEBREW LETTER DALET
U+FB23ﬣ HEBREW LETTER WIDE HE: U+05D4ה HEBREW LETTER HE
U+FB24ﬤ HEBREW LETTER WIDE KAF: U+05DBכ HEBREW LETTER KAF
U+FB25ﬥ HEBREW LETTER WIDE LAMED: U+05DCל HEBREW LETTER LAMED
U+FB26ﬦ HEBREW LETTER WIDE FINAL MEM: U+05DDם HEBREW LETTER FINAL MEM
U+FB27ﬧ HEBREW LETTER WIDE RESH: U+05E8ר HEBREW LETTER RESH
U+FB28ﬨ HEBREW LETTER WIDE TAV: U+05EAת HEBREW LETTER TAV
U+FB29﬩ HEBREW LETTER ALTERNATIVE PLUS SIGN: U+002B+ PLUS SIGN
U+1F549🕉 OM SYMBOL: U+0950ॐ DEVANAGARI OM
U+27EAF𧺯 CJK UNIFIED IDEOGRAPH-27EAF: U+FA23﨣 CJK COMPATIBILITY IDEOGRAPH-FA23

One-to-two mappings

U+0344◌̈́ COMBINING GREEK DIALYTIKA TONOS: U+0308◌̈ COMBINING DIAERESIS, U+0301◌́ COMBINING ACUTE ACCENT

U+2103℃ DEGREE CELSIUS: U+00B0° DEGREE SIGN, U+0043C LATIN CAPITAL LETTER C
U+2109℉ DEGREE FAHRENHEIT: U+00B0° DEGREE SIGN, U+0046F LATIN CAPITAL LETTER F
U+222C∬ DOUBLE INTEGRAL: U+222B∫ INTEGRAL, U+222B∫ INTEGRAL

U+2254≔ COLON EQUALS: U+003A: COLON, U+003D= EQUALS SIGN
U+2255≕ EQUALS COLON: U+003D= EQUALS SIGN, U+003A: COLON
U+2A75⩵ TWO CONSECUTIVE EQUALS SIGNS: U+003D= EQUALS SIGN, U+003D= EQUALS SIGN

One-to-three mappings

U+222D∭ TRIPLE INTEGRAL: U+222B∫ INTEGRAL, U+222B∫ INTEGRAL, U+222B∫ INTEGRAL
U+2A74⩴ DOUBLE COLON EQUAL: U+003A: COLON, U+003A: COLON, U+003D= EQUALS SIGN
U+2A76⩶ THREE CONSECUTIVE EQUALS SIGNS: U+003D= EQUALS SIGN, U+003D= EQUALS SIGN, U+003D= EQUALS SIGN

One-to-four mappings

U+2A0C⨌ QUADRUPLE INTEGRAL OPERATOR: U+222B∫ INTEGRAL, U+222B∫ INTEGRAL, U+222B∫ INTEGRAL, U+222B∫ INTEGRAL