What is the Dewey Decimal system? A guide for home and church libraries

The Dewey Decimal Classification is a way of arranging books by subject, so that books on the same topic sit together on the shelf. Melvil Dewey published it in 1876 for a college library, and it is still used by most public and school libraries in the United States and by many church and community libraries. Every subject has a number, and books are shelved in number order — so a reader who finds one book on gardening finds the others beside it.

Dewey Decimal Classification is a registered trademark of OCLC, which maintains the full classification and licenses it to libraries. AllMyBooks is not affiliated with OCLC. This guide explains how the numbers work; it does not reproduce the classification itself.

The first digit puts every book into one of ten broad classes. Each class is split into ten divisions by the second digit, and each division into ten sections by the third, so a three-digit number already says a good deal.

• 000s — general works: encyclopedias, reference books, computing, libraries themselves.
• 100s — philosophy and psychology.
• 200s — religion. Books about the Bible sit in the 220s, so a church library’s largest section is usually here.
• 300s — the social sciences: economics, law, education, customs, government.
• 400s — language: dictionaries, grammars, learning a language.
• 500s — science and mathematics: physics, chemistry, geology, plants, animals.
• 600s — technology and applied science: medicine, engineering, farming, cookery (food and drink is 641).
• 700s — the arts and recreation: painting, music, photography, sport and games.
• 800s — literature, by the language it was written in: American fiction is 813, English fiction 823.
• 900s — history and geography, including travel and, in many libraries, biography. The history of the United States is 973.

Digits after the decimal point narrow the subject further. 641 is food and drink; 641.5 is cooking; longer numbers go on to particular cuisines and methods. A library can stop wherever it likes — a small library might shelve every cookbook at 641.5, a large one might use six or seven decimal places to spread a big collection out.

The thing that trips most people up is order. The digits after the point are read one at a time, like a decimal, not as a whole number: 641.5 comes before 641.52, which comes before 641.6. A book at 599.75 shelves before one at 599.9, because 7 is less than 9 — even though 75 looks bigger than 9.

A Dewey number says what a book is about; a call number says exactly where the copy goes. The usual call number is the Dewey number with the first few letters of the author’s surname underneath — "641.5 CHI" for a cookbook by an author named Child — so books on the same subject then sit in author order. Many libraries add a prefix for a separate shelf, such as J for the children’s section or REF for reference books that stay in the building.

Fiction is the common exception. Many libraries shelve novels by author alone, with a label such as "F BRO" or "FIC BRO", instead of under 813 or 823, because readers look for novels by author rather than by language and period. Biographies are often shelved together under B or 92 and then by the subject’s surname, not the author’s.

Many books published in the United States print their own Dewey number. Look on the copyright page for a block headed "Library of Congress Cataloging-in-Publication Data". Near the end of it is a number such as 813/.54 — that is the Dewey number the Library of Congress assigned. Newer books label it: a line such as "Classification: LCC PS3613 … | DDC 813/.6" gives the Library of Congress number first and the Dewey number after DDC. The slash only marks where a smaller library may shorten the number; shelve it as 813.54, or as 813 if you use three digits.

For older books, or books with no such block, a public library’s online catalogue will usually show the number for the same edition. The number belongs to the subject, not to the copy, so the number for one edition is normally right for another printing of the same book.

Not always. Dewey earns its keep when a collection is mostly non-fiction, when several people need to find and reshelve books without asking, or when the shelves are open to borrowers — which is why church, school and community libraries tend to use it. A household with a few hundred books, mostly novels, is often happier shelving by author, by room, or by whatever system the family already uses.

• Use Dewey for the non-fiction and keep fiction by author — the most common compromise, and the one most public libraries make.
• Use three digits only. 641 for every cookbook and 220 for every Bible study is enough order for a small library, and it is far easier to keep up.
• Label the spines once the numbers are settled. Readers follow labels; volunteers reshelve by them.

• Library of Congress: Cataloging in Publication program ↗
• OCLC: Dewey Decimal Classification ↗
• AllMyBooks for church libraries →