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What is the difference between \big[ (or equivalently \big() and \bigl[? Is it always necessary to mention l (left) and r (right)?

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  • 1
    It's also instructive to consider the open interval notation, $]x[$ vs. $\left]x\right[$ or any of the \big versions. Jul 16, 2023 at 20:04

2 Answers 2

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\bigl declares an opening math delimiter with less horizontal spacing than the unspecified \big. \bigr defines a closing math delimiter. Using a \bigl and \bigr pair you could get the brackets or parentheses closer to the term within.

Just compare:

\documentclass{article}
\begin{document}
$\bigl[ \times \bigr]$

$\big[ \times \big]$
\end{document}

Output:

alt text

The definitions in latex.ltx are:

\def\bigl{\mathopen\big}
\def\bigm{\mathrel\big}
\def\bigr{\mathclose\big}
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    Is there a way to make this distinction without specifying a larger size? Aug 6, 2010 at 17:30
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    For normal brackets it's automatically done. Compare $[ \times ]$ to $\mathord[ \times \mathord]$.
    – Stefan Kottwitz
    Aug 6, 2010 at 17:48
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    What you're saying is that [ is automatically interpreted as an opening math delimiter, so one must manually force it not to be, if desired; is that right? Aug 6, 2010 at 19:18
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    That's true. fontmath.ltx defines: \DeclareMathDelimiter{[}{\mathopen} {operators}{"5B}{largesymbols}{"02}
    – Stefan Kottwitz
    Aug 6, 2010 at 19:45
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    Does the same spacing issue happen to [x] and \left[x\right]? Jan 8, 2015 at 0:12
26

You can see the difference in the following example. The left modifiers \bigl etc. are basically \mathopen{}\big. You also have to use \mathopen if you are using \left and \right to do automatic scaling to get correct spacing in some cases.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align}
  x &= \sin\biggl(\frac12\biggr)          \\ % good
  x &= \sin\mathopen{}\bigg(\frac12\bigg) \\ % good
  x &= \sin\bigg(\frac12\bigg)            \\ % bad
  x &= \sin\left(\frac12\right)           \\ % bad
  x &= \sin\mathopen{}\left(\frac12\right)   % good
\end{align}

\end{document}
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    I see. So what I now understand is that one should use \mathopen{} for a function, e.g., f\mathopen{}\left(x^{2^2}\right), but don’t use it for a multiplication, e.g., x \cdot \left(x^{2^2}\right) = x \left(x^{2^2}\right). It would be great if someone with some typographic experience could confirm! (Alternatively, I could open a new question.) Mar 13, 2018 at 20:26
  • It's not 100% clean when one should use either. Is \mathopen{} only for function arguments?
    – mu7z
    Jun 17, 2020 at 22:33

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