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Possible Duplicate:
How to automatically resize the vertical bar in a set comprehension?

Trying to type the formula for a conditional probability like this

\mathrm{P} \left( whatever \mid whatever \right) = ...,

I'd like to have \mid the same size as the left and right parentheses.

How can I achieve this?

marked as duplicate by Juan A. Navarro, Leo Liu, lockstep, Caramdir, Alan Munn May 7 '11 at 14:24

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use \middle| or for a double \middle|\middle|

\[ \left( \frac{a}{b^2}+1\middle| f(x)\right)\]
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    Arg, 12sec to late ... I would have used \middle\vert, but \middle| seems to do the same. – Martin Scharrer May 5 '11 at 15:07
  • I was 29 seconds too late... – Gonzalo Medina May 5 '11 at 15:11
  • @Herbert: Thanks, works fine (I just have to adjust the spaces around the | a little)! – j.p. May 5 '11 at 15:13
  • @Martin: Sorry, but you could have been a little less altruistic and first answer the question before beautifying it. – j.p. May 5 '11 at 15:15
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    @Herbert: \vert is the same as |, \Vert is the same as \| – egreg May 5 '11 at 16:04
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\middle doesn't add spaces around the symbol it produces and treats it as an ordinary symbol, like when we say \bigg| or even \bigg(. On the other hand it can't be put inside \mathrel.

The problem might arise when when \middle is used in a subscript; one can say either of

\left( something \nonscript\;\middle|\nonscript\; something \right)
\left( something \mathrel{}\middle|\mathrel{} something \right)

and the result will be the same as if we used \mid (of course if we decide that this vertical bar must be treated as a relation symbol). This is for writing macros that use the \middle feature, as in direct input one knows what space is necessary.

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