This presentation is about the history and development of the Incompatible Timesharing System (ITS), a unique operating created at the MIT AI lab in the 1960s. It ran on a handful of large 36-bit PDP-10 computers, each serving many users at MIT. The culture surrounding ITS strongly influenced the free software movement and the GNU Project. ITS was host to a number of influential programs and technologies, for example Maclisp, Scheme, Zork, Lisp machines, Macsyma, SHRDLU, Logo, Emacs, and the MacHack VI chess program.