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.