(CTSS) One of the earliest (1963) experiments in the design of interactive time-sharing operating systems. CTSS was ancestral to Multics, Unix, and ITS. It was developed at the MIT Computation Center by a team led by Fernando J. Corbato. CTSS ran on a modified IBM 7094 with a second 32K-word bank of memory, using two 2301 drums for swapping. Remote access was provided to up to 30 users via an IBM 7750 communications controller connected to dial-up modems.

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.