OpenStep is both an operating system and a class framework originally developed by NeXT, but now owned by Apple. As an OS under that title, it is based on Mach; the last released version was 4.2. Since then, it has been converted into Rhapsody by Apple, merging in a Mac OS look and feel, and is undergoing further merging into Mac OS X. The class framework part of OpenStep is currently code-named Yellow Box by Apple.