Like many things in this area, the answer is usually "You'll find out if you want to go up against an army of lawyers". The last three companies I worked for all claimed ownership of any IP I create, on or off the job, using company's equipment or using my own equipment. One of them explicitly called it out during the interview: You will have to stop working on open source or publishing side projects when working here. Can they do that? Maybe, probably not. It doesn't matter because I do not plan to bankrupt myself fighting their lawyers.