### `\@` and `@` in macro names

Macro names containing `@` are internal to LaTeX, and without special treatment just don't work in ordinary use. An exemplar 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 wrongly, you 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 LaTeX Companion are full of fragments of packages as examples for you to employ.

For example, there's a lengthy section in The Companion about `\@startsection` and how to use it to control the appearance of section titles. Page 15 discusses the problem; and suggests that you make such changes in the document preamble, between `\makeatletter` and `\makeatother`. So the redefinition of `\subsection` on page 26 could be:

      \makeatletter
      \renewcommand{\subsection}{\@startsection
        {subsection}%                    % name
        ...
        {\normalfont\normalsize\itshape}}% style
      \makeatother

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 approach is marginally preferable, from the LaTeX purist's point of view.)