system of library organization developed by the Indian librarian S.R. Ranganathan (Ranganathan, Shiyali Ramamrita) in 1933. It is general rather than specific in nature, and it can create complex or new categories through the use of facets, or colons. The category of dental surgery, for example, symbolized as L 214:4:7, is created by combining the letter L for medicine, the number 214 for teeth, the number 4 for diseases, and the number 7 for surgery.

In Colon Classification, there are 108 main classes (previously there were 33) and 10 generalized classes (broadly divided between the humanities and sciences), which are represented by a mixed notation of Arabic numerals and roman and Greek letters. Each main class comprises five fundamental facets, or groups: personality, matter, energy, space, and time. Ranganathan's main contribution to classification was the notion of these fundamental facets, or categories.

Instead of schedules of numbers for each topic, Colon Classification uses series of short tables from which component numbers are chosen and linked by colons to form a whole. The book number is an integral part of the call number, a departure from Dewey or Library of Congress systems. Each main class has its appropriate facets and focuses; e.g., literature has language and form. In addition, there are four floating tables that correspond to subdivisions—e.g., form, geography, time, and language. Further expansion of the tables is allowed through colon addition or omission (if the subject cannot be expanded).

The collection of the University of Madras (Madras, University of), India, was utilized in the creation of Colon Classification.