Subscript
A subscript is a number, figure, or indicator that appears smaller than the normal line of type and is set slightly below it. Subscripts are typically used in formulas, mathematical expressions, and descriptions of chemical compounds. One well known example is the number 2 in the formula for the molecule of water: H2O. Subscripts are also used frequently in mathematics to define different versions of the same variable; for example, in an equation x0 and xf may indicate the initial and final value of x.
In professional typography, subscript characters are not simply ordinary characters reduced in size; to keep them visually similar to the rest of the font, typeface designers make them slightly heavier than a reduced-size character would be. Likewise, the amount that subscripted text is dropped below the original baseline varies by typeface. However, a good approximation to professional appearance can be obtained by using regular characters reduced to 58–67% of their original size, dropped below the baseline by about 10–20% of the original font size. (So for 12-point type, a good chemical subscript might be 7 or 8 points in size, dropped between 1.5 and 2.5 points. The best values will depend on the typeface.)
Software Support
Many text editing and word processing programs have automatic subscripting features. The default for OpenOffice, for example, is to reduce the type to 58% of the original size and drop it by 10%; these values can also be set manually.
| HTML subscripts |
|---|
| AbCd |
In HTML and Wiki syntax, subscript text is produced by putting it inside the tags <sub> and </sub>. The exact size and position of the subscript will vary by font and browser, but is usually reduced to around 75% original size, and dropped between 15% and 50%.
In TeX's math mode (as used in MediaWiki), subscripts are typeset with the underscore. Thus $X_{ab}$ produces .
Unicode defines subscript characters 0 through 9 as well as +, −, =, (, ), a, e, o, x, and ə. Note, however, that most fonts which include these characters use them for mathematical denominator glyphs, which are subscripted but not dropped below the baseline (similarly, the superscript characters included in most fonts are usually numerator glyphs, which are aligned with the cap line). For details and examples, see Unicode subscripts and superscripts.