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The Dewey Decimal Classification (DDC), commonly called the Dewey Decimal System, is a way libraries organize materials by subject. Its numbers lead from broad subject areas to narrower ones, helping readers browse shelves and locate items through a library catalog. “Duey” is a misspelling; the standard spelling is “Dewey.”
What is the Dewey Decimal System?
The DDC is a hierarchical subject classification for organizing library collections. It groups materials according to what they are about: the number’s broad category identifies a general field, while additional digits can point to a more specific subject. The system is widely used internationally, but it is not universal, and a Dewey number is not a unique identifier for a particular book or copy.
OCLC describes the DDC as having ten main classes, each divided into ten divisions, which are further divided into sections. Its summaries therefore outline 10 main classes, 100 divisions, and 1,000 sections. The hierarchy offers room for increasingly specific subjects, but not every possible division or section number is used. OCLC Support’s main-classes overview
What do the Dewey Decimal numbers mean?
The first digit identifies one of the ten broad classes. Further digits narrow the subject within that class. Think of the number as a subject address within the classification: it helps indicate where a topic belongs, rather than uniquely identifying an edition or an individual item.
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For example, a book about a particular subject may be shelved near other works assigned to a similar part of the classification. The exact number and shelf location depend on the catalog record and the library’s local practices. Use the library’s catalog or ask staff when you need to find a specific item.
What are the ten main classes?
OCLC’s introduction lists these broad subject groupings. More specific numbers subdivide each class.
| Class | Broad subject |
|---|---|
| 000 | Computer science, information, and general works |
| 100 | Philosophy and psychology |
| 200 | Religion |
| 300 | Social sciences |
| 400 | Language |
| 500 | Science |
| 600 | Technology |
| 700 | Arts and recreation |
| 800 | Literature |
| 900 | History and geography |
These are broad starting points, not a complete list of the subjects or numbers a library might use. OCLC notes that the 000 class includes disciplines concerned with knowledge and information, while 900 is primarily history and geography. OCLC’s introduction to the DDC
How does the system help you find a library item?
- Search the library catalog. Look up the title, author, or subject to find the item’s catalog record.
- Check its location and call number. The catalog’s record tells you whether the item is available and provides the shelf information used by that library.
- Follow the library’s shelf arrangement. A Dewey number can guide you to a subject area, but local arrangements and the item’s catalog record determine where to look.
Not every library uses Dewey for every collection or arranges materials in the same way. Treat the number as one part of the locating information, not a substitute for the catalog.
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Yes. OCLC says libraries in more than 135 countries use DDC or systems based on it; its page does not state a publication date for that figure. OCLC’s list of countries with libraries that use the DDC
OCLC maintains professional tools and editions for applying the classification. It describes WebDewey as the electronic version of the DDC, with published numbers, mappings, and approved new terms. The 2026 print-on-demand edition includes changes and topics available as of January 2026; OCLC says the print version is updated annually. OCLC WebDewey · OCLC announcement about the 2026 print-on-demand edition
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