cls­guide – Doc­u­men­ta­tion of LaTeX class and pack­age writ­ing

Doc­u­men­ta­tion of com­mands (other than font-han­dling com­mands) that are use­ful when writ­ing a LaTeX class or pack­age.
Part of a set of doc­u­men­ta­tion that also cov­ers new LaTeX 2e com­mands, font-han­dling, type­set­ting Cyril­lic, LaTeX font en­cod­ings, con­fig­u­ra­tion op­tions and mod­i­fi­ca­tion of la­tex.