Nicholas Bodley wrote:

>Well, it's hardly worth all that introduction, but if we don't have an
>inverted caret, and a [v] probably doesn't have the right implications,
>somebody chose to use [_], a bit too low to be a lowered dash (or is it?),
>but the suggestion of something either low, or being moved down, seemed
>sensible. "H_2O" was the sort of notation used in a formal academic paper
>I came across recently. It does seem that a different markup would be
>needed to indicate more than one consecutive subscripted character,
>though. I like it. :)
>
>

The underscore is used to indicate subscripting in the typesetting
program TeX, which many computer programmers are familiar with.
Sometimes you'll even catch them using TeX-specific peculiarities, e.g.
Writing H_2O but C_{10}H_{22}O_{11}, because TeX only subscripts the
next element, which is just the next single number, unless it is
enclosed in braces. (which is why H_2O requires no end-of-subscript
marker).

TeX convention is frequently seen in geeky correspondence, since TeX can
represent pretty much anything--though not always transparently.

~mark