As soon as a book is classified enter it at once in your shelf-list--explained in a later chapter--and see that an author-card for it is put in the author catalog--explained later--with its proper number thereon. If, after you have made up your mind, from an examination of the title-page, or table of contents, or a few pages here and there, what subject a book treats of in the main, you are still in doubt in what class to place it, consider what kind of readers will be likely to ask for it, and in what class they will be likely to look for it, and put it into that class. In doubtful cases the catalogs of other libraries are often good guides. Keep your classification as consistent as possible. Before putting a book, about which there is any opportunity for choice, in the class you have selected for it, examine your shelf-list and see that the books already there are of like nature with it. Classify as well as you can, and don't worry if you find you have made errors. There are always errors. Don't get into the habit of changing. Be consistent in classifying, and stick by what you have done. CHAPTER XXI The Dewey or Decimal system of classification [From the Introduction to the Decimal classification and Relative index. Published by the Library Bureau, $5.] 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.