OPAC stands for Online Public Access Catalog, which is an electronic database that replaced the card catalog in libraries. It is an online bibliography of a library collection that is available to the public. Users can search for materials held by a library or group of libraries through the OPAC. The first large-scale online catalogs were developed in the 1970s and 1980s, and they have largely replaced the analog card catalogs previously used in libraries. With the arrival of the internet, most libraries have made their OPAC accessible from a server to users all over the world. The Z39.50 protocol is used for user searches of an OPAC, and this protocol can also be used to link disparate OPACs into a single "union" OPAC.