Books are classified by bringing together those which have the same characteristics. [6] Of course any characteristics might be taken, as size, or binding, or publisher. But as nobody wants to know what books there are in the library in folio, or what quartos, or what books bound in russia or calf, or what published by John Smith, or by Brown, Jones, and Robinson, these bases of classification are left to the booksellers and auctioneers and trade sales. Still, in case of certain unusual or noted bindings, as human skin or Grolier’s, or early or famous publishers, as Aldus and Elzevir, a partial class-list is sometimes very properly made. But books are most commonly brought together in catalogues because they have the same authors, or the same subjects, or the same literary form, or are written in the same language, or were given by the same donor, or are designed for the same class of readers. When brought together because they are by the same author, they are not usually thought of as classified; they form the author-catalogue, and need no further mention here except in regard to arrangement. The classes, i. e., in this case the authors, might of course be further classified according to their nations, or their professions (as the subjects are in national or professional biographies), or by any other set of common characteristics, but for library purposes an alphabetical arrangement according to the spelling of their names is universally acknowledged to be the best.
The classification by language is not generally used in full. There are catalogues in which all the English books are separated from all the foreign; in others there are separate lists of French books or German books. The needs of each library must determine whether it is worth while to prepare such lists. It is undeniably useful in almost any library to make lists of the belles lettres in the different languages; which, though nominally a classification by language, is really a classification by literary form, the object being to bring together all the works with a certain national flavor—the French flavor, the German flavor, or it may be a classing by readers, the German books being catalogued together for a German population, the French for the French, and so on. Again, it is useful to give lists not of the belles lettres alone, but of all the works in the rarer languages, as the Bodleian and the British Museum have published separate lists of their Hebrew books. Here too the circumstances of each library must determine where it shall draw the line between those literatures which it will put by themselves and those which it will include and hide in the mass of its general catalogue. Note, however, that some of the difficulties of transliterating {10} names of modern Greek, Russian authors, etc., are removed by putting their original works in a separate catalogue, though translations still remain to puzzle us.
The catalogue by donors or original owners is usually partial (as those of the Dowse, Barton, Prince, and Ticknor libraries). The catalogues by classes of readers are also partial, hardly extending beyond Juvenile literature and Sunday-school books. Of course many subject classes amount to the same thing, the class Medicine being especially useful to medical men, Theology to the theologians, and so on.
Classification by subject and classification by form are the most common. An example will best show the distinction between them. Theology, which is itself a subject, is also a class, that is, it is extensive enough to have its parts, its chapters, so to speak (as Future Life, Holy Spirit, Regeneration, Sin, Trinity), treated separately, each when so treated (whether in books or only in thought) being itself a subject; all these together, inasmuch as they possess this in common, that they have to do with some part of the relations of God to man, form the class of subjects Theology. Class, however, is applied to Poetry in a different sense. It then signifies not a collection of similar subjects, but a collection of books resembling one another in being composed in that form and with that spirit, whatever it is, which is called poetical. In the subject-catalogue class it is used in the first sense—collection of similar subjects; in the form-catalogue it is used in the second—list of similar books.
Most systems of classification are mixed, as the following analysis of one in actual use in a small library will show:
| Art, science, and natural history. | Subj. |
| History and biography. | Subj. |
| Poetry. | Form (literary). |
| Encyclopædias and books of reference. | Form (practical). |
| Travels and adventures. | Subj. (Has some similarity to a Form-class.) |
| Railroads. | Subj. |
| Fiction. | Form. (Novels, a subdivision of Fiction, is properly a Form-class; but the differentia of the more extensive class Fiction is not its form, but its untruth; imaginary voyages and the like of course imitate the form of the works which they parody.) |
| Relating to the rebellion. | Subj. |
| Magazines. | Form (practical). |
| General literature, essays, and religious works. | A mixture: 1. Hardly a class; that is to say, it probably is a collection of books having only this in common, that they will not fit into any of the other classes; 2. Form; 3. Subj. |
Confining ourselves now to classification by subjects, the word can be used in three senses:
1. Bringing books together which treat of the same subject specifically.
That is, books which each treat of the whole of the subject and not of a part only.
2. Bringing books together which treat of similar subjects.