Z39.50 is an international industry standard that specifies a client/server protocol for searching and retrieving information from remote databases. It refers to the International Standard, ISO 23950: "Information Retrieval (Z39.50): Application Service Definition and Protocol Specification", and to ANSI/NISO Z39.50. The Library of Congress is the Maintenance Agency and Registration Authority for both standards, which are technically identical (with minor editorial differences). For more information on Z39.50, visit the Library of Congress website http://lcweb.loc.gov/z3950/agency/
Although Z39.50 is an international industry standard, there is a great deal of variation in how it is implemented. Different information providers may support different versions of the standard. Or they may support some but not all of the possible attributes. For example, they may support Keyword searches, but not Subject searches.
When searching with Z39.50, your search results will vary depending on which attributes you search and whether or not the information provider you’re searching supports the same attributes.