Subscript

Subscript is a character or numeral that is set slightly below the baseline of a font. Subscript characters are smaller and displaced downward, used to convey chemical formulas, isotopes, mathematical indices, and other specialized notations.

In chemistry, subscripts indicate the number of atoms in a molecule; for example H2O uses subscript 2 to show two hydrogen atoms. In mathematics and computer science, subscripts serve as indices or labels for array elements or variables, as in a_i or x_n.

In typography, subscripts are typically achieved with a subscript style or markup; in HTML, the element <sub>...<sub> marks subscript text; in LaTeX, subscript is created with an underscore in math mode.

Unicode provides some subscript characters, such as digits 0–9 and a few letters, but many letters lack dedicated subscript forms; font support varies, so rendering may differ across platforms.

Accessibility considerations: screen readers generally read subscript as normal text, so semantic markup and mathematical markup (MathML, LaTeX) help preserve meaning for assistive technologies.

Subscripts should not be confused with superscripts, which are placed above the baseline.