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

LaTeX defines inline- and display-maths commands, apparently analagous to those that derive from the TeX primitive maths sequences bracketing maths commands with single dollar signs (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 `\``]` 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 `\``[``...` `\``]` whenever unadorned displayed maths is needed in LaTeX.

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