macOS first came out in 2001, and is completely different than the "classic" Mac OS that it replaced. Unlike that old (original) operating system, macOS (since OS X) is based on the Unix operating system (current versions have UNIX 03 certification) and on technologies developed between 1985 and 1997 at NeXT, a company that Apple co-founder Steve Jobs created after leaving Apple in 1985. The "X" in Mac OS X and OS X is the Roman numeral for the number 10 and is pronounced as such. The core of macOS is an open source OS called Darwin, but Darwin itself cannot run macOS software.