Synonyms: language operating systems (language-OSs), language-OS hybrids, language-OS combinations, language-integrated OSs, language-centric OSs, language-based OSs, language-dependent OSs. In computer science, after basic hardware, language comes first, before OSs, or applications. One needs a language first, even if only machine language, before one can write an OS or any other program. This category features languages that have the properties of OSs, and do OS tasks and services; and OSs intentionally built on a foundation of some specific language. This is usually done to increase system consistency, clarity, some type(s) of efficiency, programming productivity, and/or compactness. The key issue is language dependency. If an OS exists because of, is based and depends on, has a design informed by, one language, then it belongs here. If an OS is language independent, and would exist or continue regardless of its language, thought it may be coded in only one language (usually Assembly or C), then it belongs elsewhere. At the boundary, where so many arguments live, this is sometimes a difficult decision, and persons of goodwill can disagree. On this page, language-OSs are arranged in two groups and levels: 1) Top group: language-OSs for which there are more than one instance of a language-OS of this name/type, a language-OS family. 2) Bottom group: specific language-OSs, individual instances; there is only one language-OS of this name/type.