classes – The source of LaTeX's standard classes
The article, report and book classes of current LaTeX (and their class option files) all derive from a single source file, classes.dtx; its contents are extracted to the class files themselves in the course of installing LaTeX.
Of course, since classes.dtx is a documented source file, it may be formatted, for (perhaps) more relaxed reading. A formatted version exists in the base and latex-tds zip files of the LaTeX-TDS distribution.
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components (or support) of TeX distribution