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I was reading the mathtools documentation on \DeclarePairedDelimiter, and apparently only the starred version of the command it defines uses \left and \right on the delimiters.

This surprised me a little bit, because I've always used \left and \right on every single \newcommand I define which uses delimiters, I just figured there's no reason not to. Which brings me to the question.

Is there a reason not to always use \left and \right on every pair of delimiter in your equations?

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At some point I was automatically generating TeX code from some language-specific syntax. I initially made all my delimiters \left-\right pairs, but stopped because they prevent line breaks. It isn't always a good idea to have line breaks in inline equations, but in my case it was a lesser evil compared with having grossly underfilled lines.

As mentioned in comments, other reasons not to use \left and \right include their sometimes overeager spacing, both horizontal and vertical. I would say that \left and \right are a good start, but then you'll want to tweak some cases manually and use \big and friends instead.

\left and \right need to appear on the same line in multiline equations, but that's easily solved by putting \right. at the end of the initial line and \left. at the beginning of the final line.

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    \left( .. \right. \\ \left. .. \right) will only work properly if the highest part of the equation on the first line is about the same height as that on the second line, otherwise the parenthesis won't have the same size. – Torbjørn T. Jan 2 '14 at 22:36

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