About Dewey
Introduction   |   Frequently Asked Questions   |   DDC 21 Summaries   |   DDC Bibliography   |   
DDC Bibliography 2000-2001 Update   |    Melvil Dewey Biography   |   Dewey Tutorials

Introduction to the Dewey Decimal Classification

History and Current Use

The Dewey Decimal Classification (DDC) system is a general knowledge organization tool that is continuously revised to keep pace with knowledge. The system was conceived by Melvil Dewey in 1873 and first published in 1876. The DDC is published by Forest Press, a division of OCLC Online Computer Library Center, Inc. OCLC owns all copyright rights in the Dewey Decimal Classification.

The Dewey Decimal Classification is the most widely used classification system in the world. Libraries in more than 135 countries use the DDC to organize and provide access to their collections, and DDC numbers are featured in the national bibliographies of sixty countries. Libraries of every type apply Dewey numbers on a daily basis and share these numbers through a variety of means (including WorldCat, the OCLC Online Union Catalog). Dewey is also used for other purposes, e.g., as a browsing mechanism for resources on the Web.

The DDC has been translated into over thirty languages. Translations of the latest full and abridged editions of the DDC are completed or underway in Arabic, Chinese, French, Greek, Hebrew, Icelandic, Italian, Korean, Norwegian, Russian, and Spanish.

Development

One of Dewey's great strengths is that the system is developed and maintained in a national bibliographic agency, the Library of Congress. The Dewey editorial office is located in the Decimal Classification Division of the Library of Congress, where annually the classification specialists assign over 110,000 DDC numbers to records for works cataloged by the Library. Having the editorial office within the Decimal Classification Division enables the editors to detect trends in the literature that must be incorporated into the Classification. The editors prepare proposed schedule revisions and expansions, and forward the proposals to the Decimal Classification Editorial Policy Committee (EPC) for review and recommended action.

EPC is a ten-member international board whose main function is to advise the editors and OCLC Forest Press on matters relating to changes, innovations, and the general development of the Classification. EPC represents the interests of DDC users; its members come from public, special, and academic libraries, and from library schools.

Editions

The Dewey Decimal Classification is published in two editions, full and abridged. The Classification is kept up-to-date electronically through electronic versions: Dewey for Windows, a CD-ROM product that is updated annually and released in January; and WebDewey in CORC, a Web-based product that is updated quarterly. We also post selected updates to the Classification on the OCLC Forest Press Web site <http://www.oclc.org/fp>: new and changed entries on the first day of each month, and LC subject headings--selected from recent LC Subject Headings Weekly Lists and accompanied by candidate DDC numbers from Edition 21--every two weeks. These updates are incorporated into Dewey for Windows and WebDewey each time those products are released.

Structure and Notation

The DDC is built on sound principles that make it ideal as a general knowledge organization tool: meaningful notation in universally recognized Arabic numerals, well-defined categories, well-developed hierarchies, and a rich network of relationships among topics. In the DDC, basic classes are organized by disciplines or fields of study. At the broadest level, the DDC is divided into ten main classes, which together cover the entire world of knowledge. Each main class is further divided into ten divisions, and each division into ten sections (not all the numbers for the divisions and sections have been used). The three summaries of the DDC follow this introduction.

The first summary contains the ten main classes. The first digit in each three-digit number represents the main class. For example, 500 represents natural sciences and mathematics.

The second summary contains the hundred divisions. 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 summary contains the thousand sections. 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.

Arabic numerals are used to represent each class in the DDC. A decimal point follows the third digit in a class number, after which division by ten continues to the specific degree of classification needed.

A subject may appear in more than one discipline. 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.

Hierarchy

Hierarchy in the DDC is expressed through structure and notation. Structural hierarchy means that all topics (aside from the ten main classes) are part of all the broader topics above them. Any note regarding the nature of a class holds true for all the subordinate classes, including logically subordinate topics classed at coordinate numbers.

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
			636.7	Dogs
			636.8	Cats

"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."

Sometimes, other devices must be used to express the hierarchy when it is not possible or desirable to do so through the notation. Relationships among topics that violate notational hierarchy are indicated by special types of headings, notes, and entries.

Arrangement of the DDC

The print version of Edition 21 is composed of nine major parts in four volumes as follows:

Volume 1:
(A)	New Features: A brief explanation of the special features and changes in Edition 21
(B)	Introduction: A description of the DDC and how to use it
(C)	Glossary: Short definitions of terms used in the DDC
(D)	Index to the Introduction and Glossary
(E)	Tables: Seven numbered tables of notation that can be added to class numbers to provide greater specificity:
	T1	Standard Subdivisions
	T2	Geographic Areas, Historical Periods, Persons
	T3	Subdivisions for the Arts, for Individual Literatures, for Specific Literary Forms
		T3-A	Subdivisions for Works by or about Individual Authors
		T3-B	Subdivisions for Works by or about More than One Author
		T3-C	Notation to Be Added Where Instructed in Table 3-B, 700.4, 791.4, 808–809
	T4	Subdivisions of Individual Languages and Language Families
	T5	Racial, Ethnic, National Groups
	T6	Languages
	T7	Groups of Persons
	Except for notation from Table 1 (which may be added to any number unless there is an instruction in the schedules or tables to the contrary), table notation may be added only as instructed in the schedules and tables
(F)	Lists that compare the previous edition with the new edition: Relocations and Reductions; Comparative and Equivalence Tables; Reused Numbers
Volumes 2 and 3:
(G)	Schedules: The organization of knowledge from 000–999
Volume 4:
(H)	Relative Index: An alphabetical list of subjects with the disciplines in which they are treated subarranged alphabetically under each entry
(I)	Manual: A guide to classifying in difficult areas, information on new schedules, and an explanation of the policies and practices of the Decimal Classification Division at the Library of Congress. Information in the Manual is arranged by the numbers in the tables and schedules

Entries

Entries in the schedules and tables are composed of a DDC number in the number column (the column at the left margin), a heading describing the class that the number represents, and often one or more notes. All entries (numbers, headings, and notes) should be read in the context of the hierarchy.

The first three digits of schedule numbers (main classes, divisions, sections) appear only once in the number column, when first used. They are repeated at the top of each page where their subdivisions continue. Subordinate numbers appear in the number column, beginning with a decimal point, with the initial three digits understood.

Options

Numbers and notes in parentheses provide options to standard practice. These options enable the Classification to serve needs beyond those represented in the standard English-language edition.

Unused Numbers

Numbers in square brackets represent topics that have been relocated or discontinued, or are unassigned. Square brackets are also used for standard subdivision concepts that are represented in another location. Numbers in square brackets are not used.

Number Building

Only a fraction of potential DDC numbers are included in the schedules. The classifier will often find that to arrive at a precise number for a work it is necessary to build (synthesize) a number that is not specifically printed in the schedules. There are four sources of notation from which to build numbers: (A) Table 1 Standard Subdivisions; (B) Tables 2 - 7; (C) other parts of the schedules; and (D) add tables in the schedules. Number building is initiated only upon instructions in the schedules (except for the addition of standard subdivisions, which may be added anywhere unless there is an instruction to the contrary). Number building begins with a base number (always stated in the instruction note) to which another number is added.

More Information

For more information about the Dewey Decimal Classification, consult the introduction in volume 1 of Edition 21 (or the DDC Introduction in the online versions). A more in-depth discussion of the DDC may be found in Dewey Decimal Classification: A Practical Guide, 2nd ed., by Lois Mai Chan, John P. Comaromi, Joan S. Mitchell, and Mohinder Satija (Albany, N.Y.: OCLC Forest Press, 1996). The OCLC Forest Press Web site contains up-to-date information about the DDC, licensing opportunities, and OCLC Forest Press publications.