In the 1960s, Massachusetts Institute of Technology, AT&T Bell Labs, andGeneral Electric developed an experimental operating system called Multics for the GE-645 mainframe. Multics introduced many innovations, but had many problems.

Bell Labs, frustrated by the size and complexity of Multics but not the aims, slowly pulled out of the project. Their last researchers to leave Multics, Ken Thompson, Dennis Ritchie, M. D. McIlroy, and J. F. Ossanna, decided to redo the work on a much smaller scale. At the time, Ritchie says "What we wanted to preserve was not just a good environment in which to do programming, but a system around which a fellowship could form. We knew from experience that the essence of communal computing, as supplied by remote-access, time-shared machines, is not just to type programs into a terminal instead of a keypunch, but to encourage close communication."

-- from Wikipedia