A taller version of \parallel

Question

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...

this is a variant of the question Automatic left and right commands. — barbara beeton, Oct 06 '13 at 14:38

Max · Accepted Answer · 2013-10-07T09:15:45.717

10

\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.

edited Oct 07 '13 at 09:15

answered Oct 06 '13 at 03:48

Max

5,702

1

\rVert would be better on the right side. – barbara beeton Oct 06 '13 at 12:35

score 8 · Answer 2 · edited Apr 13 '17 at 12:34

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}

output of example code

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.

score 5 · Answer 3 · answered Oct 06 '13 at 04:03

5

The simpliest way:

\[
\left\| x \right\|
\]

answered Oct 06 '13 at 04:03

Przemysław Scherwentke

37,268

A taller version of \parallel

3 Answers3