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This question relates to one about getting a vertical bar longer than \mid.
How to get a vertical bar which is longer than \mid ?
However while the given \middle| solution works for me, it no longer behaves as a binary relation and so loses the space to the left-and-right of it that \mid puts in.
(I know I could manually add space, but I want the right way to do this.)
In fact I want a "growable \mid" that works like \left\{ and its relatives.
If you want to get \middle| to behave exactly as \mid on all accounts (spacing, stacking in the form \mid\mid, etc.), you need to use \mathrel{} on both sides of \middle| (manual spaces like \; won't do the job correctly in all situations):
\newcommand{\relmiddle}[1]{\mathrel{}\middle#1\mathrel{}}
Here's a code that compares \mid and the new command \relmiddle on two different formulas and shows that the spacing is identical:
\documentclass{article}
\newcommand{\relmiddle}[1]{\mathrel{}\middle#1\mathrel{}}
\begin{document}
\nulldelimiterspace=0pt% to kill the little space before \left and after \right (not a good idea in a real document)
$a \mid -b$\par
$\left. a \relmiddle| -b \right.$
$a \mid\mid -b$\par
$\left. a \relmiddle|\relmiddle| -b \right.$
\end{document}
\relmiddle|\relmiddle|, it may be better to type \relmiddle\Vert; the latter approach makes for slightly tighter (and probably more appropriate) spacing between the vertical bars. In fact, \relmiddle\Vert has the exact same look (and spacing properties) as does \parallel.
(Edit: This answer was based on the mistaken assumption that the question was about a binary operator, not a relation. Philippe's answer is better for the latter, but it is not quite so easy if you want a binary operator that grows. See also the comments.)
My answer is like Will's (now gone), except a bit more elaborate:
\def\binmiddle#1{\mathchoice{\mskip\medmuskip}{\mskip\medmuskip}{}{}\middle#1
\mathchoice{\mskip\medmuskip}{\mskip\medmuskip}{}{}}
Explanation: A binary operator is surrounded by \medmuskip in display and text mode, but no space in script and scriptscript mode.
\medmuskip roughly translates as "with a stingy mouse".
Nov 17, 2010 at 10:27
\mathchoice by using \nonscript: \nonscript\mskip\medmuskip. But the solution isn't correct any more with LuaTeX, where the spacing around a Rel atom might be different front \medmuskip. I think Philippe's solution is both more robust (since it always works) and more idiomatic (you say what you want, not how to achieve it).
\mathrel) like mid is surrounded by \thickmuskip; it's binary operations (\mathbin, like +, \times, etc.) which use \medmuskip. The difference is not really visible a lot, but can be in a context with strong inter-word stretching or shrinking (\medmuskip shrinks a lot but not \thickmuskip; here, that's not a real concern because there is no stretching/shrinking inside a \left...\right construct).
Nov 19, 2010 at 12:14