\
[
...\
]
in place of $$
...$$
?\
(
...
\
)
, 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