The field of knowledge is divided into nine main classes, and these are numbered by the digits 1 to 9. Cyclopedias, periodicals, etc., so general in character as to belong to no one of these classes, are marked nought, and form a tenth class. Each class is similarly separated into nine divisions, general works belonging to no division having nought in place of the division number. Divisions are similarly divided into nine sections, and the process is repeated as often as necessary. Thus 512 means Class 5 (Natural science), Division 1 (Mathematics), Section 2 (Algebra), and every algebra is numbered 512.

The books on the shelves and the cards in the subject catalog are arranged in simple numerical order, all class numbers being decimals. Since each subject has a definite number, it follows that all books on any subject must stand together. The tables show the order in which subjects follow one another. Thus 512 Algebra precedes 513 Geometry, and follows 511 Arithmetic.

In the book after the tables of the classes arranged in their numerical order is an index, in which all the heads of the tables are arranged in one simple alphabet, with the class number of each referring to its exact place in the preceding tables. This index includes also, as far as they have been found, all the synonyms or alternative names for the heads, and many other entries that seem likely to help a reader find readily the subject sought. Though the user knows just where to turn to his subject in the tables, by first consulting the index he may be sent to other allied subjects, where he will find valuable matter which he would otherwise overlook.

The claims of the system may be summed up as follows: compared with other systems it is less expensive; more easily understood, remembered, and used; practical rather than theoretical; brief and familiar in its nomenclature; best for arranging pamphlets, sale duplicates, and notes, and for indexing; susceptible of partial and gradual adoption without confusion; more convenient in keeping statistics and checks for books off the shelves; the most satisfactory adaptation of the card catalog principle to the shelves. It requires less space to shelve the books; uses simpler symbols and fewer of them; can be expanded, without limit and without confusion or waste of labor, in both catalogs and on shelves, or in catalogs alone; checks more thoroughly and conveniently against mistakes; admits more readily numerous cross references; is unchangeable in its call-numbers, and so gives them in all places where needed, as given in no other system; in its index affords an answer to the greatest objection to class catalogs, and is the first satisfactory union of the advantages of the class and dictionary systems.

The Decimal system is used by a large number of libraries in this country, and has gained recognition and has been put to use by some librarians and men of science in Europe.