mod­guide – The rules about mod­i­fy­ing LaTeX

A dis­cus­sion of the rules for mod­i­fy­ing LaTeX, and the ra­tio­nale for those rules.
Part of a set of doc­u­men­ta­tion that also cov­ers new LaTeX 2e com­mands, font-han­dling, class and pack­age writ­ing, type­set­ting Cyril­lic and LaTeX font en­cod­ings, con­fig­u­ra­tion op­tions.