# What is the correct way to do delimiters?

I've always used \left and \right with my delimiters, but Martin Heller's answer to another question indicates that this method gives incorrect spacing. The correction appears to be prepending \mathopen{}. Is this the canonical "correct" method? And what exactly does the \mathopen command do? A quick Google search does not turn up much useful information on it.

The other alternative offered involved the \biggl and \biggr commands and their ilk. I've never quite felt comfortable with using these because it doesn't feel right to specify these sizes manually. Is there a reason to prefer these commands to \left and \right?

This question addresses the spacing problems with \left and \right and the relationship with \mathopen and \mathclose.

From the TeXBook:

At this point you are probably wondering why should you bother learning about \bigl and \bigr and their relatives, when \left and \right are there [...] There are at least three situations in which you will want to use your wisdom [...]:

1. Sometimes \left and \right use a smaller delimiter than you want. For example, [...] $\left|\left|x\right|-\left|y\right|\right|$.
2. Sometimes \left and \right choose a larger delimiter than you want. For example, [...] $$\left( \sum_{k=1}^n A_k \right)$$.
3. Sometimes you need to break a huge formula into two or more separate lines, and you want to make sure that its opening and closing delimiters have the same size; but you can't use \left on the first line and \right on the last, since \left and \right must occur in pairs.

[...] Of course, one of the advantages of \left and \right is that they can make arbitrarily large delimiters.

Well, in most cases \left and \right are not needed. Often the size of delimiters does not need to be changed. For example $(a+b)$ is enough and gives the same result as $\left(a+b\right)$. There are many cases where \left and \right choose the wrong sizes, for example $\left(\sum_{i=1}^\infty ... \right)$ gives most likely far to big delimiters, better is to select the appropriate size yourself. $\Bigl( \sum ...\Bigr)$ is probably what I would do (I just finished typing my PhD thesis in maths and have not used a single time the \left or \right command).

\mathopen belongs with \mathord, \mathop, \mathbin, \mathrel, \mathclose and \mathpunct to a family of commands which changes the classes of individual characters or whole subformulas in mathmode. More information can be found in Knuth's "The TeXbook", p. 154ff.

• I agree, typically \left and \right don't do the right thing; you'll need to fine-tune the typesetting manually by using something like \Bigl, \biggl, etc. – Jukka Suomela Aug 11 '10 at 22:10

Martin's answer is specifically about spacing following so called 'large operators'. Those are operators like \int and \sum, but also the goniometric functions \sin, \cos, et cetera. The special thing about those is that these operators are followed by a delimited argument. In all other cases, \left and \right work just fine.

• Hmm interesting. Can you explain a bit more what 'goniometric functions' are? Does this apply to \Pr? – Neil Olver Aug 10 '10 at 14:56
• In English, "goniometric" functions are more commonly known as trigonometric functions. I don't think that this is a TeXnical term :) – José Figueroa-O'Farrill Aug 10 '10 at 23:46
• I was going to post this as a separate question, but probably better here so there's no duplication: so the rule of thumb is that one should use \bigl,\bigr for operators and \left, \right for everything else, correct? What is used for enclosing arrays? – user914 Aug 20 '10 at 22:54
• Use (the equivalent of, in a macro) \sin\mathopen{}\left( but otherwise \left( shouldn't have spacing problems. – Will Robertson Sep 2 '10 at 1:11