UK TeX FAQ -- question label dolldoll

### Why use `\``[` …`\``]` in place of `$$` … `$$`?

LaTeX defines inline- and display-maths commands, apparently duplicating the TeX primitive maths sequences which surround maths commands with single (or pairs of) dollar signs.

In fact, LaTeX’s inline maths grouping, `\``(``...` `\``)`, has (almost) exactly the same effect as the TeX primitive version `$ ... $`. (The exception: the LaTeX version checks to ensure you don’t put `\``(` and `\``)` the wrong way round; this does occasionally detect errors….)

Since this is the case, one often finds LaTeX users, who have some experience of using Plain TeX, merely assuming that LaTeX’s display maths grouping `\``[``...` `\``]` may be replaced by the TeX primitive display maths `$$ ... $$`.

Unfortunately, the assumption is wrong: some LaTeX code needs to patch display maths, it can only do so by patching `\``[` and `\``]` (or their equivalents). Most obviously, the class option `fleqn` simply does not work for equations coded using `$$ ... $$`, whether you’re using the standard classes alone, or using package amsmath. Also, the `\``[` and `\``]` construct has code for rationalising vertical spacing in some extreme cases; that code is not provided `$$ ... $$`, so if you use the Plain TeX version, you may occasionally observe inconsistent vertical spacing. Similar behaviour can bite if you are writing a proof; placing the “QED symbol” doesn’t work if it is in `$$`-displayed maths.

There are more subtle effects (especially with package amsmath), and the simple rule is “use `\``[``...` `\``]` (at least) whenever displayed maths is needed in LaTeX”.

(Note that the sequence `\``[``...` `\``]` is duplicated by the `displaymath` environment, which can be said to “look nicer”, and actually describes what’s being done.)

This answer last edited: 2013-06-05

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=dolldoll