CTSS

CTSS, which stood for the Compatible Time-Sharing System, was an operating system for the IBM 7094 mainframe computer and was originally used by Project MAC at MIT. CTSS was first published and operated in a time-sharing environment in 1961 and was one of the first modern computer operating systems.

Multics, which was also developed by Project MAC, in a loose sense was developed as sort of a successor to CTSS for future development of multiple-access computing during the 1960s. Multics, infamously, was the operating system that led to the development of Unix in 1970.