As a side effect, subscripts often cause lines to be unevenly spaced.
Let us form the sum of all x<SUB>i</SUB>'s, ie x<SUB>1</SUB> + x<SUB>2</SUB> + ... + x<SUB>n</SUB>.
Usage in chemistry:
SO<sub>3</sub> + H<sub>2</sub>O -> H<sub>2</sub>SO<sub>4</sub>
Using SUB and SUP to affect the presentation of fractions:
Fractions ½ and ¼ and ¾ have their own symbols in ISO Latin 1. Other fractions like <SUP>2</SUP>/<SUB>3</SUB> must be essentially presented in linearized notation, although you can use SUB and SUP to affect the presentation.
Since this element is new, support for it is not universal.
Some browsers simply ignore it, displaying e.g.
And naturally, text-only browsers cannot truly support SUB.
Subscripts can be nested. This may, however, result e.g. in rendering inner superscripts in a very small font. Internet Explorer ignores SUB tags after nesting level of two.
See also general notes on text markup.