= Subscript
{title2=$x_i$}
A \b[subscript] is a component placed below the usual baseline of a base expression, commonly at a smaller size. It can label a term of a <sequence>, as in $x_i$, or identify a <matrix> entry, as in $a_{ij}$. Its position does not by itself determine the mathematical meaning of the label.
In <TeX>, `x_i` attaches a single-token subscript, while `x_{i+1}` groups a compound subscript. In <MathML>, `msub` supplies a base and subscript as separate children. Their layout uses baseline shifts and clearances rather than putting both components on the ordinary <text baseline>.<MathML Core by World Wide Web Consortium>[Subscripts and superscripts: msub child layout and shift calculations]{ref}
For example, $x_{i+1}$ keeps $i+1$ together as one attached component. A <mathematical typesetting style> controls the treatment of the smaller expression. An <OpenType MATH table> can supply the font-specific placement data. This is why adding an underscore as an ordinary printed <glyph> would not reproduce the layout.
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