The library is first divided into nine special libraries which are called Classes. These Classes are Philosophy, Theology, &c., and are numbered with the nine digits. Thus Class 9 is the Library of History; Class 7, the Library of Fine Art; Class 2, the Library of Theology. These special libraries or Classes are then considered independently, and each one is separated again into nine special Divisions of the main subject. These Divisions are numbered from 1 to 9 as were the Classes. Thus 59 is the 9th Division (Zoology) of the 5th Class (Natural Science). A final division is then made by separating each of these Divisions into nine Sections which are numbered in the same way, with the nine digits. Thus 513 is the 3d Section (Geometry) of the 1st Division (Mathematics) of the 5th Class (Natural Science). This number, giving Class, Division, and Section, is called the Classification or Class Number, and is applied to every book or pamphlet belonging to the library. All the Geometries are thus numbered 513, all the Mineralogies 549, and so throughout the library, all the books on any given subject bear the number of that subject in the scheme. Where a 0 occurs in a class number, it has its normal zero power. Thus, a book numbered 510, is Class 5, Division 1, but no Section. This signifies that the book treats of the Division 51 (Mathematics) in general, and is not limited to any one Section, as is the Geometry, marked 513. If marked 500, it would indicate a treatise on Science in general, limited to no Division. A zero occurring in the first place would in the same way show that the book is limited to no Class. The classification is mainly made by subjects or content regardless of form; but it is found practically useful to make an additional distinction in these general treatises, according to the form of treatment adopted. Thus, in Science we have a large number of books treating of Science in general, and so having a 0 for the Division number. These books are then divided into Sections, as are those of the other Classes according to the form they have taken on. We have the Philosophy and History of Science, Scientific Compends, Dictionaries, Essays, Periodicals, Societies, Education, and Travels,--all having the common subject, NATURAL SCIENCE, but treating it in these varied forms. These form distinctions are introduced here because the number of general works is large, and the numerals allow of this division, without extra labor for the numbers from 501 to 509 would otherwise be unused. They apply only to the general treatises, which, without them, would have a class number ending with two zeros. A Dictionary of Mathematics is 510, not 503, for every book is assigned to the most specific head that will contain it, so that 503 is limited to Dictionaries or Cyclopedias of Science in general. In the same way a General Cyclopedia or Periodical treats of no one class, and so is assigned to the Class 0. These books treating of no special class, but general in their character, are divided into Cyclopedias, Periodicals, etc. No difficulty is found in following the arithmetical law and omitting the initial zero, so these numbers are printed 31, 32, etc., instead of 031, 032, etc.
The selection and arrangement of the thousand headings of the classification cannot be explained in detail for want of space. In all the work, philosophical theory and accuracy have been made to yield to practical usefulness. The impossibility of making a satisfactory classification of all knowledge as preserved in books, has been appreciated from the first, and nothing of the kind attempted. Theoretical harmony and exactness has been repeatedly sacrificed to the practical requirements of the library or to the convenience of the department in the college. As in every scheme, many minor subjects have been put under general heads to which they do not strictly belong. In some cases these headings have been printed in a distinctive type, e. g., 429 Anglo-Saxon, under ENGLISH PHILOLOGY. The rule has been to assign these subjects to the most nearly allied heads, or where it was thought they would be most useful. The only alternative was to omit them altogether. If any such omission occurs, it is unintentional and will be supplied as soon as discovered. Wherever practicable the heads have been so arranged that each subject is preceded and followed by the most nearly allied subjects and thus the greatest convenience is secured both in the catalogues and on the shelves. Theoretically, the division of every subject into just nine heads is absurd. Practically, it is desirable that the classification be as minute as possible without the use of additional figures, and the decimal principle on which our scheme hinges allows nine divisions as readily as a less number. This principle has proved wholly satisfactory in practice though it appears to destroy proper co-ordination in some places. It has seemed best in our library to use uniformly three figures in the class number. This enables us to classify certain subjects very minutely, giving, for example, an entire section to Chess. But the History of England has only one section, as our scheme is developed, and thus the two might be said to be co-ordinated. The apparent difficulty in such cases is entirely obviated by the use of a fourth figure, giving nine sub-sections to any subject of sufficient importance to warrant closer classification. In history where the classification is made wholly by countries, a fourth figure is added to give a division into periods. As the addition of each figure gives a ten-fold division, any desired degree of minuteness may be secured in the classing of special subjects. The apparent lack of co-ordination arises from the fact that only the first three figures of these more important heads are as yet printed, the fourth figure and the sub-sections being supplied on the catalogues in manuscript. Should the growth of any of these sub-sections warrant it, a fifth figure will be added, for the scheme admits of expansion without limit.
The arrangement of headings has been sometimes modified to secure a mnemonic aid in numbering and finding books without the Index. For instance, the scheme is so arranged that China has always the number 1. In Ancient History, it has the first section, 931: in Modern History, under Asia, it has 951: in Philology, the Chinese language appears as 491. After the same manner the Indian number is 2; Egyptian, 4; English, 2; German, 3; French, 4; Italian, 5; Spanish, 6; European, 4; Asian, 5; African, 6; North American, 7; South American, 8; and so for all the divisions by languages or countries. The Italian 5, for instance, will be noticed in 35, 55, 450, 755, 850, and 945. This mnemonic principle is specially prominent in Philology and Literature and their divisions, and in the form distinctions used in the first 9 sections of each class. Materials, Methods, or Theory occurring anywhere as a head, bears always the number 1. Dictionaries and Cyclopedias, 3; Essays, 4; Periodicals, 5; Associations, Institutions, and Societies, 6; Education, 7; Collections, 9. In the numerous cases where several minor heads have been grouped together under the head Other, it always bears the number 9. Wherever practicable, this principle is carried out in sub-dividing the sections. For instance, the Geology of North America, which bears the number 557 is sub-divided by adding the sections of 970 (History of North America). The Geology of Mexico then bears the number 5578: mnemonically, the first 5 is the Science number; the second 5, Geology; the 7, North America; and the 8, Mexico. Any library attendant or reader after using the scheme a short time will recognize at a glance, any catalogue or ledger entry, book or pamphlet, marked 5578 as something on the Geology of Mexico. Users of the scheme will notice this mnemonic principle in several hundred places in the classification, and will find it of great practical utility in numbering and finding books without the aid of Catalogue or Index, and in determining the character of any book simply from its call number as recorded on the book, on all its catalogue and cross reference cards, on the ledger, and in the check box.
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.
In arranging the books on the shelves, the absolute location by shelf and book number is wholly abandoned, the relative location by class and book number being one of the most valuable features of the plan. The class number serves also as the location number and the shelf number in common use is entirely dispensed with. Accompanying the class number is the book number, which prevents confusion of different books on the same subject. Thus the first Geometry catalogued is marked 513-1; the second 513-2, and so on to any extent, the last number showing how many books the library has on that subject. The books of each section are all together, and arranged by book numbers, and these sections are also arranged in simple numerical order throughout the library. The call number 513-11 signifies not the 11th book on shelf 513; or alcove 5, range 1, shelf 3, as in most libraries, but signifies the 11th book in subject 513 or the 11th Geometry belonging to the library. In finding the book, the printed numbers on the backs are followed, the upper being the class and the lower the book number. The class is found in its numerical order among the classes as the shelf is found in the ordinary system: the book in its numerical order in the class. The shelves are not numbered, as the increase of different departments, the opening of new rooms, and any arrangement of classes to bring the books most circulated nearest to the delivery desk, will bring different class numbers on a given shelf. New books as received are numbered and put into place, in the same way that new titles are added to the card catalogue.
The single digit occasionally prefixed to the book number, e.g. the 3 in 421-3-7 is the nearest height in decimeters of books too large to be put on the regular library shelves, which are only 2½ decimeters apart. The great mass of the library consists of 2-decimeter books, the size numbers of which are omitted. Books from 2½ to 3½ decimeters in height have 3 prefixed to the book number, and are found on the bottom shelf of each range. The larger sizes are prefixed with 4, 5, &c., and are found on the special shelves provided, in order to avoid the great waste of space otherwise occasioned by the relative location. By this use of the size numbers a close economy of space is secured.
Thus all the books on any given subject are found standing together, and no additions or changes ever separate them. Not only are all the books on the subject sought, found together, but the most nearly allied subjects precede and follow, they in turn being preceded and followed by other allied subjects as far as practicable. Readers not having access to the shelves find the short titles arranged in the same order on the Shelf Catalogue, and the full titles, imprints, cross references, notes, &c., on the Subject Catalogue. The uncatalogued pamphlets treating of any subject bear the same class number and are arranged on the shelves immediately after the books of each section.