Melvil Dewey (1851-1931) anonymously published a system in 1876 used by most school and small public libraries of classifying library books. His classifications divide nonfiction books into 10 broad categories:

000-099 General works (encyclopedias and similar works)

100-199 Philosophy (how people think and what they believe

200-299 Religion (including mythology and religions of the world)

300-399 Social sciences (folklore and legends, government, manners and customs, vocations)

400-499 Language (dictionaries, grammars)

500-599 Pure science (mathematics, astronomy, chemistry, nature study)

600-699 Technology (applied sciences - aviation, building, engineering, homemaking)

700-799 Arts (photography, drawing, painting, music, sports)

800-899 Literature (plays, poetry)

900-999 History (ancient and modern, geography, travel)

Each of these sections is further divided for accuracy in classification. For example, the numbers 500-599 cover the pure sciences, such as astronomy, chemistry, mathematics, paleontology, and physics. Each of these areas has its own division and section number. All books on mathematics are assigned numbers in the 510 to 519 range; mathematics is then broken down into types, such as algebra, arithmetic, and geometry. Geometry's specific number is 513, which can be subdivided with decimal points to provide 10 basic categories. Additional digits can be added to create even more precise categorization.

Books are arranged alphabetically within each classification by the first letters of the author's last name. Therefore, a library that has several books on American history of the colonial period will assign the same basic number (973.2) to all the books and shelve them alphabetically.

Dewey's aim was to create a system that would be simple enough for even casual users to understand, but complex enough to meet a library's expanding needs. His system was developed to meet the needs of many libraries. A second popular system was created to fit the requirements of a specific library, the Library of Congress. This system, now in wide use, is even more detailed and has the advantage of adapting growth of knowledge in unexpected areas.