CTSS

/CTSS/ , n. Compatible Time-Sharing System. An early (1963) experiment in
the design of interactive timesharing operating systems, ancestral to
Multics , Unix , and ITS. The name ITS (Incompatible Time-sharing System)
was a hack on CTSS, meant both as a joke and to express some basic
differences in philosophy about the way I/O services should be presented to
user programs.