MacOS was first released in 1999 as Mac OS X Server 1.0. To ease the transition for users and developers, versions through 10.4 were able to run Mac OS 9 and its applications in the Classic Environment, a compatibility layer.

The transition was a technologically and strategically significant one.

Īlthough it was originally marketed as simply 'version 10' of Mac OS (indicated by the Roman numeral 'X'), it has a completely different codebase from Mac OS 9, as well as substantial changes to its user interface.

However, the current macOS is a UNIX operating system built on technology that had been developed at NeXT from the 1980s until Apple purchased the company in early 1997. That system, up to and including its final release Mac OS 9, was a direct descendant of the operating system Apple had used in its Mac computers since their introduction in 1984. The history of macOS, Apple's current Mac operating system formerly named Mac OS X until 2011 and then OS X until 2016, began with the company's project to replace its 'classic' Mac OS.

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