Source : Free On-Line Dictionary of Computing

Compatible Timesharing System
     
         (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.
     
        (1997-01-29)