Mac OS X (originally called Rhapsody) is the industrial strength OS produced by Apple to run on PowerPC systems (replacing what is often referred to as Mac OS Classic. Mac OS X is at its heart a variant of UNIX and possesses its underlying power (and the ability to run many of the traditional UNIX tools, including the GNU tools). It also was designed to mimic other OSes on demand via what it originally refered to as "boxes" (actually high-performance emulators); it has the built-in capability to run programs written for older Mac OS (via its "BlueBox", officially called Mac OS Classic) and work was started on making it also run Windows '95 / '98 / ME software (via what was called its "YellowBox"). There are also a few rumors going around that future versions may even be able to run Newton software (via the "GreenBox"). It provides a selection of two window managers built-in: Aqua and X-Windows (with Aqua being the default).