# Lower vertical space in \left[ \right]

When using more than one mathematical accent on top of a symbol inside \left[ \right], the symbol appears in the vertical centre of the brackets, leaving an empty space in the lower part of the symbol. Here is a MWE

\documentclass{article}
\usepackage{bm}

\newcommand{\sub}[1]{{\scriptscriptstyle\mathrm{#1}}} %Small Math Sub
\newcommand{\Cov}[1]{\mathrm{Cov}\left[#1\right]}

\begin{document}
$\left[\vec{\bm{s}}_\sub{n}\right]$

$\left[\hat{\vec{\bm{s}}}_\sub{n}\right]$

$\Cov{\vec{\bm{s}}_\sub{n}}$

$\Cov{\hat{\vec{\bm{s}}}_\sub{n}}$

$\Cov{\tilde{\bm{A}}_\sub{L_\sub{n}}}$
\end{document}


This extra space is a problem e.g. when inserting a math expression inline with text.

How can one remove the vertical empty space under the symbol so that the lower end of the brackets are aligned with it?

You can use \vcenter

\documentclass{article}
\usepackage{amsmath,bm}

\newcommand{\sub}[1]{{\scriptscriptstyle\mathrm{#1}}} %Small Math Sub

\begin{document}
$\left[\vec{\bm{s}}_\sub{n}\right]$

$\left[\vcenter{\hbox{$\hat{\vec{\bm{s}}}_\sub{n}$}}\right]$

$\bigl[\vcenter{\hbox{$\hat{\vec{\bm{s}}}_\sub{n}$}}\bigr]$
\end{document}


Or

\documentclass{article}
\usepackage{mathtools,bm}

\newcommand{\sub}[1]{{\scriptscriptstyle\mathrm{#1}}} %Small Math Sub
\DeclarePairedDelimiterX\mybracket[1]{[}{]}{\vcenter{\hbox{$#1$}}}

\begin{document}
$\mybracket{\vec{\bm{s}}_\sub{n}}$

$\mybracket{\hat{\vec{\bm{s}}}_\sub{n}}$

$\mybracket{\hat{\vec{\bm{s}}}_\sub{n}}$

$\mybracket[\big]{\hat{\vec{\bm{s}}}_\sub{n}}$   %% optional argument can be size

$\mybracket*{\hat{\vec{\bm{s}}}_\sub{n}}$    %% starred version uses \left and \right
\end{document}

• Put the symbols next to each other and you'll see that the “s” doesn't sit on the baseline any more. – egreg Nov 1 '15 at 14:36
• @egreg I agree. This was a solution in a hurry. – user11232 Nov 2 '15 at 5:17

There's no need that fences cover entirely the symbols between them. Don't use \left and \right unless really necessary and in this case they aren't.

\documentclass{article}
\usepackage{amsmath,bm}

\newcommand{\sub}[1]{{\scriptscriptstyle\mathrm{#1}}} %Small Math Sub

\begin{document}
$[\vec{\bm{s}}_\sub{n}]$
$[\,\hat{\!\vec{\bm{s}}}_\sub{n}]$
$\bigl[\,\hat{\!\vec{\bm{s}}}_\sub{n}\bigr]$
\end{document}


Note the usage of amsmath that improves a bit the placement of the hat, which however needs some more help.

• The particular MWE does not need \left and \right, but is a simplification of a more complex problem. In reality I define new commands like \newcommand{\Cov}[1]{\mathrm{Cov}\left[#1\right]} which I use in many places with different arguments. Thus I need that the size of the brackets is "adaptive" (see the edited MWE) – user24528 Nov 1 '15 at 21:17
• @user24528 I can't see where \Cov needs \left and \right in those examples. Look for \DeclarePairedDelimiter in the manual of mathtools` for better “adaptive” fence macros. – egreg Nov 1 '15 at 21:22