 macOS is based on the Unix operating system 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 X was a prominent part of the operating system's brand identity and marketing in its early years, but gradually receded in prominence since the release of Snow Leopard in 2009.