# A taller version of \parallel

I need a taller version of \parallel in order to typeset a norm correctly. I have tried variations of things like

\Bigg{\parallel}


and

\left{\parallel} \right{\parallel}


but receive the error "Missing delimiter (. inserted)" with the file not compiling. Any suggestions would be appreciated...

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this is a variant of the question Automatic left and right commands. –  barbara beeton Oct 6 '13 at 14:38

\parallel isn't defined as a delimiter which is necessary to work with \left and \right. To achieve the desired effect try:

$\left\lVert \frac a b\right\rVert$


This needs amsmath, so make sure you load it.

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\rVert would be better on the right side. –  barbara beeton Oct 6 '13 at 12:35
Thank you, obviously you are right. –  Max Oct 7 '13 at 9:16

if you're using the norm notation frequently, you might want to define a command \norm using the \DeclarePairedDelimiter feature of mathtools:

\documentclass{article}
\usepackage{mathtools}

\DeclarePairedDelimiter\norm{\lVert}{\rVert}

\begin{document}
$\norm{a} + \norm[\bigg]{\frac{1}{a}}$
\end{document}


this "helper" command provides the ability to specify a size option as shown, and a starred version automatically applies \left and \right. mathtools automatically loads amsmath, loading that separately isn't necessary.

a more complete explanation of how to automatically provide \left and \right commands for paired delimiters is given by this question, and the present answer is a variant on the answer to that question by lev bishop.

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The simpliest way:

$\left\| x \right\|$

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