Originally posted in reply to How Linux could have be inspired by Unix if it was a closed source OS? on Reddit
AT&T licensed Unix, including source code to several companies and universities. Some of those licensees wrote replacements for the AT&T components of the system and released them as free software. Eventually enough of those components were rewritten that they could run a working system without any AT&T code. Minix was one such system. Linus was inspired by Minix but chose to develop a monokernel, which was Linux.