It has seemed best in our library to use uniformly three figures in the class number.  This enables us to classify certain subjects very minutely, giving, for example, an entire section to Chess.  But the History of England has only one section, as our scheme is developed, and thus the two might be said to be co-ordinated.  The apparent difficulty in such cases is entirely obviated by the use of a fourth figure, giving nine sub-sections to any subject of sufficient importance to warrant closer classification.  In history where the classification is made wholly by countries, a fourth figure is added to give a division into periods.  As the addition of each figure gives a ten-fold division, any desired degree of minuteness may be secured in the classing of special subjects.  The apparent lack of co-ordination arises from the fact that only the first three figures of these more important heads are as yet printed, the fourth figure and the sub-sections being supplied on the catalogues in manuscript.  Should the growth of any of these sub-sections warrant it, a fifth figure will be added, for the scheme admits of expansion without limit.

The arrangement of headings has been sometimes modified to secure a mnemonic aid in numbering and finding books without the Index.  For instance, the scheme is so arranged that China has always the number 1.  In Ancient History, it has the first section, 931:  in Modern History, under Asia, it has 951:  in Philology, the Chinese language appears as 491.  After the same manner the Indian number is 2; Egyptian, 4; English, 2; German, 3; French, 4; Italian, 5; Spanish, 6; European, 4; Asian, 5; African, 6; North American, 7; South American, 8; and so for all the divisions by languages or countries.  The Italian 5, for instance, will be noticed in 35, 55, 450, 755, 850, and 945.  This mnemonic principle is specially prominent in Philology and Literature and their divisions, and in the form distinctions used in the first 9 sections of each class.  Materials, Methods, or Theory occurring anywhere as a head, bears always the number 1.  Dictionaries and Cyclopedias, 3; Essays, 4; Periodicals, 5; Associations, Institutions, and Societies, 6; Education, 7; Collections, 9.  In the numerous cases where several minor heads have been grouped together under the head Other, it always bears the number 9.  Wherever practicable, this principle is carried out in sub-dividing the sections.  For instance, the Geology of North America, which bears the number 557 is sub-divided by adding the sections of 970 (History of North America).  The Geology of Mexico then bears the number 5578:  mnemonically, the first 5 is the Science number; the second 5, Geology; the 7, North America; and the 8, Mexico.  Any library attendant or reader after using the scheme a short time will recognize at a glance, any catalogue or ledger entry, book or pamphlet, marked 5578 as something on the Geology of Mexico.  Users of the scheme will notice this mnemonic principle in several hundred places in the classification, and will find it of great practical utility in numbering and finding books without the aid of Catalogue or Index, and in determining the character of any book simply from its call number as recorded on the book, on all its catalogue and cross reference cards, on the ledger, and in the check box.