The AthenaMuse environment attempts to offer true application portability across the UNIX and Windows platforms. It currently runs on three flavors of UNIX (SunOS 4.2.n, Solaris 2.5, and HPP-UX 9) as well as on Win95 and Windows/NT 3.5.1. There are two key principles of AM2's approach to platform portability: Since application portability is a goal, the use of platform specific features is discouraged, but not forbidden. That is, AM2 should not deny the developer access to platform-specific features, but neither should it encourage their use. By default, an application running on a UNIX version of AM2 should obey the Motif look and feel, while a version running under Windows should obey the Windows user interface guidelines. The preliminary Macintosh version followed the same principles. The later feature makes the former less bothersome. AthenaMuse classes attempt to embody semantic functionality rather than low-level feature sets. For instance, the AM2 approach to menus is platform independent. It is the registration of the menu with a particular application on a particular operating system that determines the visual style of the menu (e.g., pull-down or pop-up, tear-off, etc.).