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I'm defining \norm by \|#\|. However, when I type \norm{x}_{M}, the M comes out at exactly the same height as if one typed x_M. The problem is really noticeable with upper-case subscripts. Is there a way to modify the \norm definition so that the subscript M is lowered slightly below \| and truly appears as its subscript?

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3 Answers

up vote 5 down vote accepted

If you want to define it yourself, it is better with the help of \DeclarePairedDelimiter from mathtools:

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

MWE

\documentclass{article}
\usepackage{mathtools}
\DeclarePairedDelimiter{\norm}{\lVert}{\rVert}
\begin{document}
$\norm{x}_Mx_M$
\end{document} 

Output

enter image description here

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Is there a variable that sets how high/low a subscript is placed? –  Chrystomath Jan 17 at 7:17
    
@Chrystomath Not that I know –  karlkoeller Jan 17 at 7:35
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Is this what you want?

\newcommand{\norm}[1]{\left\|#1\right\|} 

a$\norm{x}_Mx_M$a

enter image description here

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I hate re-inventing (Because I am lazy). With physics package:

\documentclass{article}
\usepackage{physics}
\begin{document}
  \[
  a\norm{x}_{M}a
  \]
  \[
  a\abs{x}_{M}a
  \]
\end{document}

enter image description here

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2  
+1. Thanks for introducing me physics. –  Sigur Jan 16 at 13:57
1  
@Sigur Thanks. I am a physics teacher ;-) –  Harish Kumar Jan 16 at 14:04
1  
Nice to meet you. I'm a mathematician. –  Sigur Jan 16 at 14:09
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