“Unicode subscripts and superscripts”的意思、由来-开放百科全书

Unicode has subscripted and superscripted versions of a number of characters including a full set of Arabic numerals.^[1] These characters allow any polynomial, chemical and certain other equations to be represented in plain text without using any form of markup like HTML or TeX.

The World Wide Web Consortium and the Unicode Consortium have made recommendations on the choice between using markup and using superscript and subscript characters: "When used in mathematical context (MathML) it is recommended to consistently use style markup for superscripts and subscripts.... However, when super and sub-scripts are to reflect semantic distinctions, it is easier to work with these meanings encoded in text rather than markup, for example, in phonetic or phonemic transcription."^[2]

Uses

The intended use^[2] when these characters were added to Unicode was to allow chemical and algebra formulas and phonetics to be written without markup, but produce true superscripts and subscripts. Thus "H₂O" (using a subscript character) is supposed to be identical to "H₂O" (with subscript markup).

In reality most fonts that include these characters ignore the Unicode definition, and design the digits for mathematical numerator and denominator glyphs, which are smaller than normal characters but are aligned with the cap line and the baseline, respectively. When used with the solidus, these glyphs are useful for making arbitrary diagonal fractions (similar to the ½ glyph). Trying to make fractions using existing software super/subscripts look messier (example: ¹/₂), so font designers provided this alternative. This also makes the superscript letters useful for ordinal indicators, more closely matching the ª and º characters. However it makes them incorrect for normal super and subscripts, and generally formulas look better using markup than these characters.

Unicode intended to produce diagonal fractions through a different mechanism but it is very poorly supported. The fraction slash U+2044 is visually similar to the solidus, but when used with the ordinary digits (not the superscripts and subscripts) is intended to tell a layout system that a fraction such as ¾ should be rendered^[3] using automatic glyph substitution^[4] for the digits. Some browsers support this^[5] but not in all fonts, a selection of fonts is shown in the below table.

Superscripts and subscripts block

The most common superscript digits (1, 2, and 3) were in ISO-8859-1 and were therefore carried over into those positions in the Latin-1 range of Unicode. The rest were placed in a dedicated section of Unicode at {{U+|2070}} to U+209F. The two tables below show these characters. Each superscript or subscript character is preceded by a normal x to show the subscripting/superscripting. The table on the left contains the actual Unicode characters; the one on the right contains the equivalents using HTML markup for the subscript or superscript.

Unicode characters
}}	}}	x²	x³	}}	}}	}}	}}	}}	x¹	}}	}}	}}	}}	}}	}}
x⁰	xⁱ			x⁴	x⁵	x⁶	x⁷	x⁸	x⁹	x⁺	x⁻	x⁼	x⁽	x⁾	xⁿ
x₀	x₁	x₂	x₃	x₄	x₅	x₆	x₇	x₈	x₉	x₊	x₋	x₌	x₍	x₎
xₐ	xₑ	xₒ	xₓ	xₔ	xₕ	xₖ	xₗ	xₘ	xₙ	xₚ	xₛ	xₜ

Simulated using `<sup>` or `<sub>` tags
}}	}}	x²	x³	}}	}}	}}	}}	}}	x¹	}}	}}	}}	}}	}}	}}
x⁰	xⁱ			x⁴	x⁵	x⁶	x⁷	x⁸	x⁹	x⁺	x⁻	x⁼	x⁽	x⁾	xⁿ
x₀	x₁	x₂	x₃	x₄	x₅	x₆	x₇	x₈	x₉	x₊	x₋	x₌	x₍	x₎
x_a	x_e	x_o	x_x	x_ə	x_h	x_k	x_l	x_m	x_n	x_p	x_s	x_t

Other superscript and subscript characters

Unicode version 11.0 also includes subscript and superscript characters that are intended for semantic usage, in the following blocks:^[1]^[6]

Latin and Greek tables

Consolidated, the Unicode standard contains superscript and subscript versions of a subset of Latin and Greek letters. Here they are arranged in order for comparison (or for copy and paste convenience). Since these characters come from different ranges, they may not be of the same size and position, depending on the typeface:

Composite characters

Primarily for compatibility with earlier character sets, Unicode contains a number of characters that compose super- and subscripts with other symbols.^[1] In most fonts these render much better than attempts to construct these symbols from the above characters or by using markup.

References

1. ^¹²{{cite web|url=https://www.unicode.org/Public/UCD/latest/ucd/UnicodeData.txt|title=UCD: UnicodeData.txt|work=The Unicode Standard|accessdate=2016-05-14}}
2. ^¹{{cite web |url=http://www.w3.org/TR/unicode-xml/#Superscripts |title=Unicode in XML and other Markup Languages |author=Martin Dürst, Asmus Freytag |date=16 May 2007 |publisher=W3C |accessdate=13 September 2010}}
3. ^{{cite web |url=http://www.w3.org/TR/unicode-xml/#Fraction |title=Fraction Slash |author=Martin Dürst, Asmus Freytag |date=16 May 2007 |publisher=W3C |accessdate=13 September 2010}}
4. ^For a general overview and technical information on glyph substitution (though not specifically for fractions): [https://www.microsoft.com/typography/otspec/gsub.htm GSUB — Glyph Substitution Table] in the [https://www.microsoft.com/typography/otspec/default.htm OpenType specification] on the Microsoft Typography site.
5. ^Such as [https://www.google.com/chrome/index.html Chrome] on Windows, [https://www.mozilla.org/en-US/firefox/new/ Firefox]
6. ^{{cite web|url=https://www.unicode.org/Public/UCD/latest/ucd/Scripts.txt|title=UCD: Scripts.txt|work=The Unicode Standard|accessdate=2017-06-20}}
7. ^{{Cite web|url=http://www.unicode.org/L2/L2017/17066r-marca-registrada.pdf|title=L2/17-066R: Proposal to encode the Marca Registrada sign|date=2017-03-01|first=Eduardo Marín|last=Silva}}