OS X (previously Mac OS X) is the successor of Mac OS 9. X stands for 10 in roman numerals: http://en.wikipedia.org/wiki/Roman_numerals http://en.wikipedia.org/

OS X (previously Mac OS X) is the successor of Mac OS 9. X stands for 10 in roman numerals: http://en.wikipedia.org/wiki/Roman_numerals http://en.wikipedia.org/wiki/Roman_numerals For marketing reasons (as some don't know about roman numerals), Apple writes it like this OS X 10.9 or now OS X 10.10 or OS X Yosemite (instead of Mac OS X.10 or Mac OS 10.10).