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( <expression> \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
\left\{ x \in \mathbb{N} \middle| x \mbox{ even} \right\}
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 braket package provides commands
for set specifications (as above) and for Dirac brackets (and bras and
kets). The package uses the e-TeX built-in command if it finds
itself running under e-TeX.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=braket