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Since the mid-1980s, many libraries have allowed library users to search their online catalogs over the Internet themselves, one library at a time. If they find an item they want, they have to travel to that library, and apply for a library card in order to borrow the item. By taking advantage of interlibrary loan, in comparison, the technical services department of the library can search thousands of libraries at once, transport the item from thousands of miles away, and allow them to borrow the item using their local library card. Loans between branch libraries in the same system may take one or two days, while loans between library systems may take a week or more to be delivered. However, if an item is rare or difficult to find, this may be the only way to gain access to it at all.
Libraries have formed voluntary associations with each other to provide a union catalog of all the items held by all member libraries, to facilitate interlibrary loans. OCLC (Online Computer Library Center) is usually used by public libraries in the U.S., and RLIN (Research Libraries Information Network) is usually used by academic libraries, although some libraries are members of both. Libraries that do not belong to a network participate in interlibrary loan by making the arrangements by postal mail.