One of the few glaring omissions from TeX’s mathematical
typesetting capabilities is a means of setting separators in the
middle of mathematical expressions. TeX provides primitives called
`\left`

and `\right`

, which can be used to modify brackets (of
whatever sort) around a mathematical expression, as in:
`\left(`

` `

`\right)`

— the size of the
parentheses is matched to the vertical extent of the expression.
However, in all sorts of mathematical enterprises one may find oneself
needing a `\middle`

command, to be used in expressions like

to specify the set of even natural numbers. The e-TeX system defines just such a command, but users of Knuth’s original need some support. Donald Arseneau’s\left\{ x \in \mathbb{N} \middle| x \mbox{ even} \right\}

- braket.sty
- braket

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=braket