From LQWiki

CTSS (Compatible Time-Sharing System) was 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.

This article is based, in whole or in part, on entry or entries in the Jargon File.

• Jargon File entry on CTSS