X Window, X11 or "X," as it is known for short, provides the programming framework and the underlying runtime system for most Unix and Linux-based network-transparent windowing implementations. It runs on a huge number of Linux and Unix flavors, including Mac OS X and with a bit of help, on several Windows varieties. Without X11 , there is no KDE, no GNOME, and no Linux-based window manager, unless one is prepared to accept an X replacement. They do exist, and many carry a proprietary license, while X comes with a GPL-compatible license.