UK TeX FAQ -- question label dolldoll

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

LaTeX defines inline- and display-maths commands, apparently s those that derive from the TeX primitive maths sequences bracketing maths commands with single (or pairs of) dollar signs.

As it turns out, LaTeX’s inline maths grouping, `\``(``...` `\``)`, has precisely the same effect as the TeX primitive version `$ ... $`. (Except that the LaTeX version checks to ensure you don’t put `\``(` and `\``)` the wrong way round.)

In this circumstance, 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, they are wrong: if LaTeX code is going to patch display maths, it can only do so by patching `\``[` and `\``]`. The most obvious way this turns up, is that 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 available in `$$ ... $$`, so if you use the non-standard version, you may occasionally observe inconsistent vertical spacing .

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

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