Why is \[ … \] preferable to $$ … $$?

Question

I've heard that you should use \[ ... \] for displayed equations instead of $$ ... $$, but why is that?

I'd assumed that it's so that you can more easily tell which are starting and which are ending delimiters, but if I always use a syntax-highlighting text editor, I can see that easily based on the color of the symbols.

Is there any reason for this suggestion?

Answer 1

Spacing is wrong.

It is a "deadly sin" according to l2tabu: http://www.ctan.org/tex-archive/info/l2tabu/english/

Answer 2

Here is one more reason (this is what converted me). If you use the \begin{proof}...\end{proof} environment, you occasionally end a proof with an equation. The automatic QED box then ends up on a line by itself, which is ugly. You can fix this by writing \qedhere at the end of the equation, but this doesn't work properly with $$...$$ – you have to use \[...\] (or an environment, like equation/gather/align). To exemplify:

Don't:

\begin{proof}
  This follows from
  $$e^{x+y}=e^xe^y. \qedhere$$
\end{proof}

Do:

\begin{proof}
  This follows from
  \[e^{x+y}=e^xe^y. \qedhere\]
\end{proof}

Answer 3

$$ is TeX primitive syntax, which, as others have commented is hard to redefine (in classic TeX there is no command name which triggers entering or leaving display math).

LaTeX doesn't officially support $$. The most noticeable failure if you use the syntax is that the fleqn option will no longer affect the display of the mathematics, it will remain centered rather than being set flush left.

However if you are using \[ it is best to use the amsmath package (it is best to load this package if doing any serious mathematical layout with LaTeX).

\[ is a short form of \begin{displaymath} which one might expect to act like an un-numbered form of \begin{equation}. The amsmath package redefines \[ to be \begin{equation*} which is exactly an un-numbered form of the equation environment as defined by that package. In the core LaTeX definition \[ has the definition

  \ifvmode
     \nointerlineskip
     \makebox[.6\linewidth]{}%
  \fi
  $$

which means that the spacing if used between paragraphs is better that a primitive $$ (which causes a blank paragraph just consisting of the indentation and parfillskip glue to be inserted before the display) but it is inconsistent with equation and always uses \abovedisplayskip rather than \abovedisplayshortskip. In the AMS versions the spacing is more consistent,

Answer 4

$$ is plain TeX and could have some side effects, also fleqn will not work anymore. Please have a look at l2tabu. In my opinion the best environment for equations is gather oder align. If you use equation you sometimes get some strange spacings.

Answer 5

[Edit: I should add the disclaimer that the following is based on just "I read it somewhere sometime ago", so I have no references to backup my claims.]

As far as I know $$ ... $$ is a TeX, amsTeX thing. LaTeX still supports it for one reason or another, but the "proper" one to use, as defined in the specifications, is \[ ... \]. All this just means that they are not promising that $$ will always work. So it is technically possible (though unlikely in the near future), that compatibility with $$ is removed from LaTeX and lots of your documents break.

As to why one is in the specifications and not the other? I have no idea.

Answer 6

1. While my memory prevents me from naming the guilty software, I have seen LaTeX syntax highlighting become confused by the $$ ... $$ construct when it parsed the \[ ... \] construct correctly.

2. Agreed, worry about some partially broken editor isn't a high priority. But, I have found that I am more than partially broken. If I've made a mess of a file, the left-hand and right-hand information in the \[ way has, on occasion, helped me figure out just what I was on about more easily than the symmetric $$ would have.

Answer 7

I suppose it could be useful if you share your files with someone else who doesn't have a syntax-highlighting text editor. Other than that, it probably doesn't matter much. Certainly if nobody else ever sees your LaTeX source files, I'd say write them however you want.

This is neither here nor there, but personally I find neither of those as clear as \begin{equation}...\end{equation}.