Frequently Asked Question List for TeX

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
ε-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 ε-TeX built-in command if it finds
itself running under ε-TeX.