Union catalogs have been created in a range of media, including book format, microform, cards and more recently, networked electronic databases.
Union catalogs are useful to librarians, as they assist in locating and requesting materials from other libraries through interlibrary loan service.
[1] This achievement has since been superseded by the creation of union catalogs in the form of electronic databases, of which the largest is OCLC's WorldCat.
[3] For academic publications, several academic search engines exist to combine the open data provided by open archives through OAI-PMH, as well as records from publishers deposited in CrossRef and other sources.
They include BASE, CORE and Unpaywall, which indexes over 20 million open access publications as of 2020.