(See Library Bureau catalog.) In a very small library the books in a given class can be distinguished one from another by writing after the class-number of each book the number of that book in its class.

If the class-mark of religion, for example, is 20, the books successively placed in that class will bear the numbers 20.1, 20.2, 20.3, etc.
Fiction should have author-numbers only.

The absence of a class-number will sufficiently distinguish it from other classes..