NeXTStep is the basis for OS X. Some technologies from NeXT are still present in OS X to this day. Obviously the GUI has had a ton of changes since then, but a lot of the underlying ideas are present. More info:
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en.wikipedia.org/wiki/NeXTSTEPquote:
Following an announcement on December 20, 1996,[8] on February 4, 1997, Apple Computer acquired NeXT for $429 million. Based upon the "OPENSTEP for Mach" operating system, and developing the NeXT API to become Cocoa, Apple created the basis for OS X,[9] and eventually, in turn, for iOS.
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en.wikipedia.org/wiki/Rh ··· _system)quote:
Rhapsody was the code name given to Apple Computer's next-generation operating system during the period of its development between Apple's purchase of NeXT in late 1996 and the announcement of Mac OS X in 1998. It consisted primarily of the OPENSTEP operating system ported to the Power Mac along with a new GUI to make it appear more Mac-like. Several existing Mac OS technologies were also ported to Rhapsody, including QuickTime and AppleSearch.
Rhapsody was split into Mac OS X Server 1.0 and "Mac OS X Developer Preview 1 and 2"
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en.wikipedia.org/wiki/Ma ··· rver_1.0»
www.guidebookgallery.org ··· acosxdp2They switched to Aqua by DP3:
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www.guidebookgallery.org ··· acosxdp3