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What does the colon symbol show in the Universal Decimal Classification?

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British Museum: Reading Room — library Reading Room of the British Museum, designed by Sidney Smirke in collaboration with Anthony Panizzi and built in the 1850s. Illustration by Smirke, from the *Illustrated London News*, 1857.

In the Universal Decimal Classification, the colon symbol (:) is used to indicate a relationship between two or more subjects.1

This technique of synthesis allows for the creation of complex notations by linking different parts of the classification schedules. For example, if the notation for iron and steel is 669.1 and the notation for sulfur is 546.22, the compound subject of sulfur in iron and steel is represented as 669.1:546.22. The colon is the most commonly used sign in this system to connect these distinct aspects of a subject.1