=232.= A classification system is a schedule or chart of knowledge arranged in some logical order according to a definite and invariable principle. It may arrange knowledge by the historical, evolutionary or some other and arbitrary principle, the choice of which is governed by the rule that the order must be that which is likely to be most serviceable to the users of the system. Special classifications, such as would be necessary for arranging a collection devoted to anthropology, or botany, or archæology, naturally arrange books by the principle that will most clearly reveal their place in the progress of the subject required; and such classifications are merely mentioned in passing. General classifications, which are the business of the average librarian, usually proceed in the historical or evolutionary order we have mentioned. Their schedules consist of a number of general headings, called main classes, which are divided by gradual steps in accordance with the principle employed until specific headings are reached. Each of the headings must be exclusive of subjects not falling into it. In order to make this schedule of subjects practicable as a method of book arrangement, it must be equipped with special “form” classes which accommodate general works, or works of so composite a character that they do not fall into any of the subject-classes; and which also accommodate such aggregates of literature as poetry, drama, essays, fiction, etc., which are arranged by the form in which the matter in them is presented, and not by the matter itself. Further, it must be equipped with a notation, or a shorthand sign for each of the subjects in the schedule, which may be written on the backs of books and in catalogues instead of the names of the subjects. And, finally, it must have an index which forms a ready key to the tables of the schedule, and is a convenient means of checking the placings of books. =233.= The theory of classification is a subject for special study, and there are rules of order, division, nomenclature, notation and indexing which it is useful for a library student to master. As the ground has been covered adequately by the text-books which are listed at the end of this, we shall do better to refer the reader to these rather than to enlarge this manual by attempting to traverse it. =234.= BIBLIOGRAPHY Brown, J. D. Library Classification and Cataloguing. 1912. Grafton. Dana, J. C. Classification. _In his_ Library Primer, p. 84. Edwards, Edward. Memoirs of Libraries, 1859, vol. ii., p. 761. Graesel, A. Classification. _In his_ Bibliothekslehre. 1902. Jast, L. S. Library Classification. _In_ Greenwood’s Year Book, 1900. p. 21. Richardson, E. C. Classification: Theoretical and Practical. 1901. Ed. 2, 1912. Scribner’s. (Contains the best bibliography.) Sayers, W. C. Berwick. Introduction to Library Classification. 1918. Grafton. ---- Canons of Classification. 1915. Grafton.