All operating systems (OSs) in this category support POSIX standards fully or partly. POSIX is an acronym for: Portable Operating System Interface for UniX. Much like TRON, POSIX is not a body of computer code that is compiled and run on some processor. Rather, it is a set of standards (IEEE 1003.1): interfaces, design guidelines, software design specifications, defining (for creating) the computer code that will become language interfaces between an OS kernel and its programs, to give compatibility when moving programs between compatible systems. POSIX is made mostly of features from BSD Unix and Unix System V.

Much like Open Source software, all POSIX standards are copyrighted (by the Institute of Electrical and Electronics Engineers, Inc., IEEE; new versions have joint copyright by IEEE and Open Group), but available for use by software developers anywhere in the world for free. Thus the OS architecture based on POSIX is an open architecture that invites and welcomes cloning and interoperability.

On this page, OSs are arranged in three groups and levels: 1) Top group: issues spanning multiple unrelated OSs. 2) Middle group: types or classes of OS. 3) Bottom group: specific OSs, individual instances; there is only one OS of this name/type.