UK TeX FAQ -- question label extrabrace

### An extra ‘`}`’??

You’ve looked at your LaTeX source and there’s no sign of a misplaced `}` on the line in question.

Well, no: this is TeX’s cryptic way of hinting that you’ve put a fragile command in a moving argument.

For example, `\``footnote` is fragile, and if we put that in the moving argument of a `\``section` command, as

>     \section{Mumble\footnote{I couldn't think of anything better}}

we get told

>     ! Argument of \@sect has an extra }.

The same happens with captions (the following is a simplification of a comp.text.tex post):

>     \caption{Energy: \[e=mc^2\]}

giving us the error message

>     ! Argument of \@caption has an extra }.

The similar (but more sensible):

>     \caption{Energy: \(e=mc^2\)}

is more tiresome, still: there’s no error when you first run the job … but there is on the second pass, when the list of figures (or tables) is generated, giving:

>     ! LaTeX Error: Bad math environment delimiter.

in the `\``listoffigures` processing.

The solution is usually to use a robust command in place of the one you are using, or to force your command to be robust by prefixing it with `\``protect`, which in the `\``section` case would show as

>     \section{Mumble\protect\footnote{I couldn't think of anything better}}

However, in both the `\``section` case and the `\``caption` case, you can separate the moving argument, as in `\``section[`moving`]{`static`}`; this gives us another standard route — simply to omit (or otherwise sanitise) the fragile command in the moving argument. So, one might rewrite the `\``caption` example as:

>     \caption[Energy: (Einstein's equation)]{Energy: \(E=mc^2\)}

In practice, inserting mathematics in a moving argument has already been addressed in LaTeX2e by the robust command `\``ensuremath`:

>     \caption{Energy: \ensuremath{E=mc^2}}

So: always look for alternatives to the `\``protect` route.

Footnotes can be even more complex; “footnotes in LaTeX section headings” deals specifically with that issue.

This answer last edited: 2012-02-09

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