Frequently Asked Question List for TeX
Programming
\@ and @ in macro names
Macro names containing @ are internal to LaTeX, and
without special treatment just don’t work in ordinary use. A nice
example of the problems caused is discussed in
“\@ in vertical mode”.
The problems users see are caused by copying bits of a class
(cls file) or
package (sty file) into a document, or by including a class or
package file into a LaTeX document by some means other than
\documentclass or \usepackage. LaTeX defines internal
commands whose names contain the character @ to
avoid clashes between its internal names and names that we would
normally use in our documents. In order that these commands may work
at all, \documentclass and \usepackage play around with the
meaning of @.
If you’ve included a file some other way (for example, using
\input), you can probably solve the problem by using the correct
command.
If you’re using a fragment of a package or class, you may well feel
confused: books such as the first edition of the
The LaTeX Companion
are full of fragments of packages as examples for you to employ.
The second edition of the Companion makes clearer how you
should use these fragments, and in addition, the code of
all the examples is now available on CTAN.
To see the technique in practice, look at the example below, from file
2-2-7.ltx in the Companion examples directory:
\makeatletter
\renewcommand\subsection{\@startsection
{subsection}{2}{0mm}%name, level, indent
{-\baselineskip}% beforeskip
{0.5\baselineskip}% afterskip
{\normalfont\normalsize\itshape}}% style
\makeatother
(That example appears on page 29 of The LaTeX Companion, second edition.)
The alternative is to treat all these fragments as a package proper,
bundling them up into a sty file and including them with
\usepackage; this way you hide your LaTeX internal code somewhere
that LaTeX internal code is expected, which often looks “tidier”.