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Documentation reference for LaTeX internal commands?

I'm working on writing a company-specific class file for reports in LaTeX and I thought I would start by reading report.cls. I'm having a difficult time finding concise definitions of the commands/macros available in class file writing. For example, what does \p@ do? What about \@plus? Where is \@compatibility defined?

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marked as duplicate by egreg, Joseph Wright Dec 9 '11 at 10:10

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up vote 4 down vote accepted

A \documentclass is loaded after some default LaTeX commands, defined in a combined source called latex.ltx (this is in addition to the roughly 900 TeX primitives and macros loaded/defined by default). Some of the macros defined in latex.ltx is done for the sake of brevity and/or consistency, like:

\newdimen\p@ \p@=1pt % this saves macro space and time
\newdimen\z@ \z@=0pt % can be used both for 0pt and 0

In terms of the compatibility condition, consider reading What's the use of the @compatibility condition?

For what it's worth, and as future reference, consider browsing through the "unpacked LaTeX catalogue on CTAN". latex.ltx is mostly built up from the sources lt*.dtx. When viewing latex.ltx, each reference used is clearly marked by the original source.

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