Originally Posted by CYF

University of California at Berkeley was able to use ATT's unix code to create the Berkeley Standard Distribution (BSD). All of the BSD operating systems are direct descendants and are unix. "Historically, BSD has been considered a branch of UNIX?"BSD UNIX", because it shared the initial codebase and design with the original AT&T UNIX operating system."

Linux was a made from scratch clone and I don't consider it a real unix.