Mac OS X is the tenth major version of Apple's operating system for Macintosh computers. Previous Macintosh operating systems were named using Arabic numerals, e.g. Mac OS 8 and Mac OS 9. The letter X in Mac OS X's name refers to the number 10, a Roman numeral. It is therefore correctly pronounced as the number 10 in this context (/tɛn/), though pronouncing the letter "X" (/ɛks/) is a common mistake for users.