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It is designed to run on Macintosh computers, having been pre-installed on all Macs since 2002.

Within the market of desktop, laptop and home computers, and by web usage, OS X is the second most widely used desktop OS after Windows.

Launched as Mac OS X in 2001, it is the successor to the "classic" Mac OS, the final release of which was OS 9 from 1999, which had been Apple's primary operating system since 1984.

The first version released was Mac OS X Server 1.0 in 1999; a desktop version, Mac OS X Cheetah followed in March 2001. Releases were named after big cats through OS X Mountain Lion; starting in 2013 with OS X Mavericks, they were named after California landmarks.

The latest software version is OS X El Capitan, which was publicly released in September 2015.

In June 2016, Apple announced the rebranding of the software to mac OS starting with the upcoming release, mac OS Sierra, giving the operating system a name similar to the names of Apple's other operating systems, i OS, watch OS, and tv OS.

OS X is based on technologies developed at Ne XT between the second half of the 1980s and Apple's purchase of the company in late 1996.