One context in which researchers used these terms was in the design of a [[computer network programming]] language called Decode-Encode Language (DEL).<ref name="rulifson"/> The purpose of this language was to accept commands from one computer (the user-host), which would return status reports to the user as it encoded the commands in network packets. Another DEL-capable computer, the server-host, received the packets, decoded them, and returned formatted data to the user-host. A DEL program on the user-host received the results to present to the user. This is a client–server transaction. Development of DEL was just beginning in 1969, the year that the [[United States Department of Defense]] established ARPANET (predecessor of [[Internet]]).
===Client-host and server-host===
''Client-host'' and ''server-host'' have subtly different meanings than ''client'' and ''server''. A host is any computer connected to a network. Whereas the words ''server'' and ''client'' may refer either to a computer or to a computer program, ''server-host'' and ''client-host'' always refer to computers. The host is a versatile, multifunction computer; ''clients'' and ''servers'' are just programs that run on a host. In the client–server model, a server is more likely to be devoted to the task of serving.
An early use of the word ''client'' occurs in "Separating Data from Function in a Distributed File System", a 1978 paper by Xerox PARC computer scientists Howard Sturgis, James Mitchell, and Jay Israel. The authors are careful to define the term for readers, and explain that they use it to distinguish between the user and the user's network node (the client).<ref>{{Cite web| title=Separating Data from Function in a Distributed File System |last1 = Sturgis |first1 = Howard E. |last2 = Mitchell |first2 = James George |last3 = Israel | first3 = Jay E. |publisher=[[PARC (company)|Xerox PARC]] |url=http://ip.com/IPCOM/000128883 |year = 1978}}</ref> By 1992, the word ''server'' had entered into general parlance.<ref>{{OEtymD|server|accessdate=30 November 2013}}</ref><ref>{{cite web |url=https://getinfo.de/app/Separating-data-from-function-in-a-distributed/id/TIBKAT%3A509976956 |title=Separating data from function in a distributed file system |author=<!--Staff writer(s); no by-line.--> |website=[[GetInfo]] |publisher=[[German National Library of Science and Technology]] |access-date=29 November 2013 |archive-url=https://web.archive.org/web/20131202233729/https://getinfo.de/app/Separating-data-from-function-in-a-distributed/id/TIBKAT%3A509976956 |archive-date=2 December 2013 |url-status=dead }}</ref>
