The shelf-list Many books can be very properly put in any one of several different classes.

In which one a given book should be placed will often be decided by noting where other like books have been placed.

Books by authors of the same name will often fall into the same class, and to each of these a different author-number must be given.

You must have at hand, then, a list of the books already classified, to see at once, in classifying the next book, what kinds of books and books by what authors are in each class.

Every book in the library, as soon as it has been classified, and has received its proper author-number, should be entered in a list in the order first of its class-number, next of its author-number.