Around 1993/1994 Apple decided that they needed a more modern version of MacOS. They announced that they were starting work on two new OperatingSystems, Copland which would be released in 1996 as MacOS 8 and Gershwin estimated to come out in 1998 as MacOS 9.
The Macintosh hardware market at the time was relatively unstable with Apple licencing MacOS to third parties and RAM prices were fluctuating wildy and as such the goals of Copland changed a number of times during development. The main feature of Copland was the !NuKernel?. This was to add PreemptiveMultitasking, MemoryProtection, SymmetricMultiProcessing and substantially improved VirtualMemory support into MacOS. Additional features of note were a fully Power PC Native OperatingSystem, robust Theme support, MultiUser support, Quickdraw3D, a new filesystem (HFS+), advanced !OpenTransport? networking, the (V-Twin) search engine and !OpenDoc?, a powerful ComponentArchitecture?.
In order to support these features, a number of old and obsolete API's were removed from MacOS, mostly relating to task management and memory management. Apple claimed that 70% of the software available at the time would run on the new OperatingSystem with the remaining software requiring source code changes.
In the end (1997) the project got a bit out of hand with the feature list being a constantly moving target and it was all taking too long to develop. Developers wanted some firm direction and Apple's management and marketing department were worried about increasing discontent in the user base. It was this with this train of thought that Apple cancelled both Copland and Gershwin, bought NeXT and announced a plan to turn NeXTStep? into the next version of MacOS which they codenamed Rhapsody.
Copland is generally considered to be a failed project but this is not entirely true. While the project as a whole was cancelled, many of the technologies and ideas were released independantly or as part of other system updates. Quickdraw3D, !OpenTransport? and !OpenDoc? were all released as freely available downloads for existing versions of MacOS and shipped with MacOS 7.5.2 and 7.5.3. Much of the Internet integration support was released in MacOS 7.6. Limited Theme support was released with MacOS 7.6 and was enhanced with MacOS 8 (codenamed Tempo) and MacOS 8.5 (codenamed Allegro) although even today MacOS Themes are not as powerful as what the Copland demos were showing. HFS+ was released with MacOS 8.1 (Codenamed Bride Of Buster). The V-twin search engine (now known as Sherlock) was released with MacOS 8.5. Parts of the !NuKernel? (this portion referred to as the !NanoKernel?) and a largely Power PC Native Finder were released as part of MacOS 8.6 (codenamed Veronica). Rudimentary MultiUser support was added in MacOS 9 (codenamed Sonata). Many smaller pieces were merged into MacOS 9.0.4 (codenamed Minuet), MacOS 9.1 (codenamed Fortissimo), MacOS 9.2 (codenamed Moonlight) and MacOS 9.2.1 (codenamed Limelight) and the rumored future update, MacOS 9.3 (codenamed Starlight). The new API developed for Copland became known as Carbon which was released progressively from MacOS 8 onwards. Other portions of the !NuKernel? and a more complete version of the Carbon API were merged into Rhapsody which was later renamed to MacOSX (codenamed Cheetah).