"To classify maps, atlases, and globes most advantageously in a collection extensively used by geographers, historians, economists, technicians or other specialists and students, it is believed that the categories to be considered, in order of importance, are: (1) Area, (2) Subject, and (3) Date, with (4) Author, frequently necessary. When these four categories are insufficient to differentiate between maps in the collection, a fifth, Title, may be used. A sixth, Physical form, must be taken into account in filing globes, relief maps, maps mounted on rigid board or equipped with sticks or rods. The area-subject classification of maps corresponds to the classification of books by subject. The addition of date and, when necessary, author (or publisher), usually completes the differentiation of each map from every other map. Thus classified, a map has a single specific filing position relative to all the other maps; the flexible classification permits unlimited intercalation of new maps in their logical positions, regardless of the chance order of accession into the collection. It is believed that the notation which has been adopted, in which area is represented by numbers, and subject is represented by letters, gains merit from this obvious separation. The classification system needs at the outset to provide the framework for inclusion of every likely map subject, though it need not give an absolute location for the less common subjects. This forms a contrast with the subject heading list, which is infinitely expansive by nature."