How can I tweak the length of the \left \right vertical lines of the cardinality of a displaystyle \bigcup?

Minimal working example:

\documentclass{article}

\usepackage{amsmath, amssymb}

\begin{document}

$$\left| \bigcup_{\substack{V \subseteq C_m \\ V \supsetneq U}} \mathcal{B}_V \right|$$

\end{document}


gives

which looks imbalanced as the vertical lines are much longer at the top than the bottom. How can I tweak this to make it look nicer?

Changing the \left and \right to \Big gives the following, but it's also not great:

Changing to \left\vert and \right\vert (as per here) is also poor:

• \biggl| and \biggr| should be enough. – egreg Apr 3 '17 at 11:51
• I'd say \Bigl/Bigr is enough here. It is clear what they fence in, and they do not dominate the expression. – daleif Apr 3 '17 at 11:51
• Also note: tex.stackexchange.com/questions/503/why-is-preferable-to ($$...$$ is not LaTeX) – daleif Apr 3 '17 at 11:52

Please observe \Bigl ... \Bigr, not \Big ... \Big pair and $ ... $, not $$ ... $$. You can choose more convenient vertical dimension from the suggested ones, however your second example looks pretty good.

\documentclass{article}

\usepackage{amsmath, amssymb}

\begin{document}

%    $$\left| $\Bigl\biggl\Biggl|\bigcup_{\substack{V \subseteq C_m \\ V \supsetneq U}} \mathcal{B}_V \Bigr\biggr\Biggr|$ %\right|$$

\end{document}

• Thank you! \Biggl| and \Biggr| is my favourite, so I'll use that. – Rebecca J. Stones Apr 3 '17 at 12:02

Delimiters are not required to cover the whole subformula, so \bigg or \Bigg might do:

\documentclass{article}

\usepackage{amsmath, amssymb}
\usepackage{mathtools} % for \DeclarePairedDelimiter

\DeclarePairedDelimiter{\abs}{\lvert}{\rvert}

\begin{document}

Wrong:
$\left| \bigcup_{\substack{V \subseteq C_m \\ V \supsetneq U}} \mathcal{B}_V \right|$
Possibly right:
$\biggl| \bigcup_{\substack{V \subseteq C_m \\ V \supsetneq U}} \mathcal{B}_V \biggr| \qquad \Biggl| \bigcup_{\substack{V \subseteq C_m \\ V \supsetneq U}} \mathcal{B}_V \Biggr|$
Easier
$\abs[\bigg]{\bigcup_{\substack{V \subseteq C_m \\ V \supsetneq U}} \mathcal{B}_V} \qquad \abs[\Bigg]{\bigcup_{\substack{V \subseteq C_m \\ V \supsetneq U}} \mathcal{B}_V}$
Overkill:
$\raisebox{-1.5ex}{% \abs[\Bigg]{% \raisebox{1.5ex}{% \displaystyle\bigcup_{\substack{V \subseteq C_m \\ V \supsetneq U}} \mathcal{B}_V% }% }% } =3$
\end{document}


Take your pick. I wouldn't choose the last one; personally, I'd go with \bigg.