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Two major families of Mac operating systems were developed by Apple Inc.
In 1984, Apple debuted the operating system that is now known as the "Classic" Mac OS with its release of the original Macintosh System Software. The system, rebranded "Mac OS" in 1997, was pre-installed on every Macintosh until 2002 and offered on Macintosh clones for a short time in the 1990s. Noted for its ease of use, it was also criticized for its lack of modern technologies compared to its competitors.
The current Mac operating system is macOS, originally named "Mac OS X" until 2012 and then "OS X" until 2016. Developed between 1997 and 2001 after Apple's purchase of NeXT, Mac OS X brought an entirely new architecture based on NeXTSTEP, a Unix system, that eliminated many of the technical challenges that the classic Mac OS faced. The current macOS is pre-installed with every Mac and receives a major update annually. It is the basis of Apple's current system software for its other devices – iOS, iPadOS, watchOS, and tvOS.
Prior to the introduction of Mac OS X, Apple experimented with several other concepts, releasing different products designed to bring the Macintosh interface or applications to Unix-like systems or vice versa, A/UX, MAE, and MkLinux. Apple's effort to expand upon and develop a replacement for its classic Mac OS in the 1990s led to a few cancelled projects, code named Star Trek, Taligent, and Copland.
Although they have different architectures, Mac operating systems share a common set of GUI principles, including a menu bar across the top of the screen; the Finder shell, featuring a desktop metaphor that represents files and applications using icons and relates concepts like directories and file deletion to real-world objects like folders and a trash can; and overlapping windows for multitasking.