Overview
The "Compatible" in the name refers to compatibility with the standard batch processing OS for the IBM 7094, the FORTRAN Monitor System (FMS). CTSS ran an unaltered copy of FMS, processing a standard batch stream, in a pseudo-virtual 7094 provided by its background facility. (The hardware was partly but not fully virtualized; see History of CP/CMS for further details.) Background FMS jobs could access tapes normally, but could not interfere with foreground time-sharing processes or the resources used to support them.
CTSS was very influential. It showed that time-sharing was viable; it fostered important new applications for computers; it had a significant influence on the next generation of time-sharing systems (notably CP/CMS); and its direct successor, Multics, pioneered many core concepts of current operating systems.
Read more about this topic: Compatible Time-Sharing System