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

$$ 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 2

Spacing is wrong.

It is a "deadly sin" according to l2tabu, section 1.6 on page 6: http://www.ctan.org/tex-archive/info/l2tabu/english/

Answer 3

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 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 or align. If you use equation you sometimes get some strange spacings.

Answer 5

It is not always preferable to use \[..\]!

The source2e.pdf file contains the documented source of LaTeX2e. Therein, one sees that \[..\] is essentially a wrapper for $$ .. $$ checking if the construct is used when already in math mode (which is then an error).

Produces $$...$$ with checks that \[ isn’t used in math mode, and that \] is only used in math mode begun with \].

There seems to be a typo there \[ was meant. Besides, the sentence should not be taken to mean that

\[ a^n + b^n= c^n $$
$$ a^n + b^n= c^n \]

create errors, they don't, only things like \( a^n + b^n= c^n \] or $a^n + b^n= c^n \] do.

However the detailed source code shows that it contains something which will interfere in some cases with correct vertical spacing:

170 \def\[{%
171 \relax\ifmmode
172 \@badmath
173 \else
174 \ifvmode
175 \nointerlineskip
176 \makebox[.6\linewidth]{}%
177 \fi
178 $$%%$$ BRACE MATCH HACK
179 \fi
180 }
181 \def\]{%
182 \relax\ifmmode
183 \ifinner
184 \@badmath
185 \else
186 $$%%$$ BRACE MATCH HACK
187 \fi
188 \else
189 \@badmath
190 \fi
191 \ignorespaces
192 }

The \makebox[.6\linewidth]{} which is executed when the display is opened in vertical mode has the effect of disabling the TeX use of \abovedisplayshortskip (hence of \belowdisplayshortskip). The problem is that immediately after \[..\] TeX is in horizontal mode (not vertical) hence if two or more equations are typeset using \[..\] without anything in-between, the second and next ones will use the short skips. This leads to incoherent vertical spacing:

\documentclass{article}
\begin{document}

The rules are smashed so take no vertical space, and are only there to
illustrate the issue (extra vertical space only between the first and the second
equation and not between the next ones).

Using \verb|\[..\]|: 

\[
\vartheta=2\pi\smash{\rule{.4pt}{4ex}}  
\]
\[
\vartheta=2\pi\smash{\rule{.4pt}{4ex}}  
\]
\[
\vartheta=2\pi\smash{\rule{.4pt}{4ex}}  
\]

Using \verb|$$..$$|:

$$
  \vartheta=2\pi\smash{\rule{.4pt}{4ex}}
$$
$$
  \vartheta=2\pi\smash{\rule{.4pt}{4ex}}  
$$
$$
  \vartheta=2\pi\smash{\rule{.4pt}{4ex}}  
$$

There is no issue if we use \verb|\[..\]| but start within a paragraph:\newline
\[
\vartheta=2\pi\smash{\rule{.4pt}{4ex}}  
\]
\[
\vartheta=2\pi\smash{\rule{.4pt}{4ex}}  
\]
\[
\vartheta=2\pi\smash{\rule{.4pt}{4ex}}  
\]
The issue arises only if the chain of \verb|\[..\]| is initiated in vertical
mode. The issue never arises when using the original \verb|$$..$$| input syntax.

\end{document}

Answer 6

[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 7

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 8

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}.