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