The ten main classes are:
000 Computer science, information & general works
100 Philosophy & psychology
300 Social sciences
700 Arts & recreation
900 History & geography
Class 000 is the most general class, and is used for works not limited to any one specific discipline, e.g., encyclopedias, newspapers, general periodicals. This class is also used for certain specialized disciplines that deal with knowledge and information, e.g., computer science, library and information science, journalism. Each of the other main classes
(100–900) comprises a major discipline or group of related disciplines.
Class 100 covers philosophy, parapsychology and occultism, and psychology.
Class 200 is devoted to religion.
Class 300 covers the social sciences. Class 300 includes sociology, anthropology, statistics, political science, economics, law, public administration, social problems and services, education, commerce, communications, transportation, and customs.
Class 400 comprises language, linguistics, and specific languages. Literature, which is arranged by language, is found in 800.
Class 500 is devoted to the natural sciences and mathematics.
Class 600 is technology.
Class 700 covers the arts: art in general, fine and decorative arts, music, and the performing arts. Recreation, including sports and games, is also classed in 700.
Class 800 covers literature, and includes rhetoric, prose, poetry, drama, etc. Folk literature is classed with customs in 300.
Class 900 is devoted primarily to history and geography. A history of a specific subject is classed with the subject.
Since the parts of the DDC are arranged by discipline, not subject, a subject may appear in more than one class. For example, “clothing” has aspects that fall under several disciplines. The psychological influence of clothing belongs in 155.95 as part of the discipline of psychology; customs associated with clothing belong in 391 as part of the discipline of customs; and clothing in the sense of fashion design belongs in 746.92 as part of the discipline of the arts.
Arabic numerals are used to represent each class in the DDC. The first digit in each three-digit number represents the main class. For example, 500 represents science. The second digit in each three-digit number indicates the division. For example, 500 is used for general works on the sciences, 510 for mathematics, 520 for astronomy, 530 for physics. The third digit in each three-digit number indicates the section. Thus, 530 is used for general works on physics, 531 for classical mechanics, 532 for fluid mechanics, 533 for gas mechanics. The DDC uses the convention that no number should have fewer than three digits; zeros are used to fill out numbers.
A decimal point, or dot, follows the third digit in a class number, after which division by ten continues to the specific degree of classification needed. The dot is not a decimal point in the mathematical sense, but a psychological pause to break the monotony of numerical digits and to ease the transcription and copying of the class number. A number should never end in a 0 anywhere to the right of the decimal point.
Notational hierarchy is expressed by length of notation. Numbers at any given level are usually subordinate to a class whose notation is one digit shorter; coordinate with a class whose notation has the same number of significant digits; and superordinate to a class with numbers one or more digits longer. The underlined digits in the following example demonstrate this notational hierarchy:
600 Technology (Applied sciences)
630 Agriculture and related technologies
636 Animal husbandry
“Dogs” and “Cats” are more specific than (i.e., are subordinate to) “Animal husbandry”; they are equally specific as (i.e., are coordinate with) each other; and “Animal husbandry” is less specific than (i.e., is superordinate to) “Dogs” and “Cats.”
Classifying with DDC
The guiding principle of the DDC is that a work is classed in the discipline for which it is intended, rather than the discipline from which the work derives. This enables works that are used together to be found together. For example, a general work by a zoologist on agricultural pest control should be classed in agriculture, not zoology, along with other works on agricultural pest control.
The First-of-Two Rule
If two subjects receive equal treatment, and are not used to introduce or explain one another, class the work with the subject whose number comes first in the DDC schedules. This is called the first-of-two rule. For example, a history dealing equally with the United States and Japan, in which the United States is discussed first and is given first in the title, is classed with the history of Japan because 952 Japan precedes 973 United States.
Sometimes, specific instructions are given to use numbers that do not come first in the schedules. For example, at 598, the note “class comprehensive works on warm-blooded vertebrates in 599” tells the classifier to ignore the first-of-two rule and class a work on birds (598) and mammals (599) in 599, which is the comprehensive number for warm-blooded vertebrates.
Also disregard the first-of-two rule when the two topics are the two major subdivisions of a subject. For example, collection systems (628.142) and distribution systems (628.144) taken together constitute 628.14 Collection and distribution systems. Works covering both of these topics are classed in 628.14 (not 628.142).