This document discusses the different forms that library catalogs can take, including physical (outer) forms like book, sheaf, and card forms, as well as logical (inner) forms like author, title, subject, dictionary, and classified catalogs. It outlines the merits and demerits of each form, such as flexibility, ease of use, cost, and ability to be updated. More recent options discussed include computerized catalogs and online public access catalogs (OPACs). The document concludes that both classified and alphabetical catalogs have advocates, and that the optimal choice depends on factors like library type and resources.