ITS (Incompatible Timesharing System) was a famous OS developed by hackers at MIT's AI lab from 1967 to the early 1980's for the DEC PDP-6 and PDP-10 family. It was famous for being the antithesis of MIT's security-obsessed CTSS (Compatible Time-Sharing System), by being very hackable and so user rights were a matter of social respect and ethics alone, rather than the dictates of the system, which often limited performance and capacity of the system. ITS was driven by the hacker ethic. Richard Stallman, founder of the GNU project, got the idea of free software from his work with ITS.