In naming the headings, brevity has been secured in many cases at the sacrifice of exactness.  It was thought more important to have short, familiar titles for the headings than that the names given should express with fullness and exactness the character of all books catalogued under them.  Many subjects, apparently omitted, will be found in the Index, assigned, with allied subjects, to a heading which bears the name of the most important only.  Reference to this Subject Index will decide at once any doubtful points.

In arranging books in the classification, as in filling out the scheme, practical usefulness has been esteemed the most important thing.  The effort has been to put each book under the subject to the student of which it would be most useful.  The content or the real subject of which a book treats, and not the form or the accidental wording of the title, determines its place.  Following this rule, a Philosophy of Art is put with Art, not with Philosophy; a History of Mathematics, with Mathematics, not with History; for the philosophy and history are simply the form which these books have taken.  The true content or subject is Art, and Mathematics, and to the student of these subjects they are most useful.  The predominant tendency or obvious purpose of the book, usually decides its class number at once; still many books treat of two or more different subjects, and in such cases it is assigned to the place where it will be most useful, and underneath the class number are written the numbers of any other subjects on which it also treats.  These Cross References are given both on the plate and the subject card as well as on the cross reference card.  If a book treats of a majority of the sections of any division, it is given the Division number instead of the most important Section number with cross references.

Collected works, libraries, etc., are either kept together and assigned like individual books to the most specific head that will contain them; or assigned to the most prominent of the various subjects on which they treat with cross references from the others; or are separated and the parts classed as independent works.  Translations are classed with their originals.

The Alphabetical Subject Index is designed to guide, both in numbering and in finding the books.  In numbering, the most specific head that will contain the book having been determined, reference to that head in the Index will give the class number to which it should be assigned.  In finding books on any given subject, reference to the Index will give the number under which they are to be sought on the shelves, in the Shelf Catalogue, or in the Subject Catalogue.  The Index gives after each subject the number of the class to which it is assigned.  Most names of countries, towns, animals, plants, minerals, diseases, &c, have been omitted, the aim being to furnish an Index of Subjects on which books are written, and not a Gazetteer or a Dictionary of all the nouns in the language.  Such subjects will be found as special chapters or sections of books on the subjects given in the Index.  The names of individual subjects of biographies will be found in the Class List of Biography.  Omissions of any of the more general subjects will be supplied when brought to notice.