Rhapsody was Apple’s initial attempt to combine Mac OS and NeXTSTEP to form the company’s next-gen operating system:
The Rhapsody Developer Release (RDR) has a fundamental incompatability with OPENSTEP/Mach (and NeXTStep) that you should be aware of when planning how to install and evaluate this software. Specifically, RDR/Intel uses a different file system format than either RDR/PPC or OPENSTEP/NeXTStep. You will not be able to set up systems to dual-boot RDR/Intel and previous Mach/Intel operating systems. You will also not be able to exchange external hard disks and/or removable media between RDR/Intel and other versions of Mach. Rhapsody/PPC will be able to read (mount read-only) media created on previous versions of Mach. Read/write access to older media is not supported.
Rhapsody would surface as a few developer builds and as the backbone of Mac OS X Server 1.0, but it would be replaced by an updated strategy that birthed OS X as we know it today.
Also, there’s a typo in the article’s title.