In a catalogue, individual bibliographic entries are organized into a coherent file. Two predominant methods of organizing the entries are the systematic or classified arrangement and the alphabetical arrangement.
In a classified catalogue, the entries are arranged according to a certain system of classification, resulting in subject collocation. This is the older form of catalog arrangement. This form of catalog as a public tool has become all but extinct in American libraries. However, as a working tool for catalogers, this form is still extant in the shelf list usually located in the cataloging department.
In an alphabetical, or dictionary catalogue, entries are organized in alphabetical sequence without regard to subject relationship. This form was introduced in the latter part of the nineteenth century and has since become the predominant form of catalog arrangement in American libraries.