Z39.50

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Z39.50 is an international standard client–server, application layer communications protocol for searching and retrieving information from a database over a TCP/IP computer network, developed and maintained by the Library of Congress. It is covered by ANSI/NISO standard Z39.50, and ISO standard 23950.

Z39.50 is widely used^{[as of?]} in library environments, for interlibrary catalogue search and loan, often incorporated into integrated library systems and personal bibliographic reference software, and social media such as LibraryThing.

Work on the Z39.50 protocol began in the 1970s, and led to successive versions in 1988, 1992, 1995 and 2003. The Contextual Query Language (formerly called the Common Query Language)^[1] is based on Z39.50 semantics.