The X.Org implementation serves as the canonical implementation of X. Due to liberal licensing, a number of variations, both free and open source and proprietary, have appeared. Commercial Unix vendors have tended to take the open source implementation and adapt it for their hardware, usually customizing it and adding proprietary extensions.
Up to 2004, XFree86 provided the most common X variant on free Unix-like systems. XFree86 started as a port of X for 386-compatible PCs and, by the end of the 1990s, had become the greatest source of technical innovation in X and the de facto standard of X development. Since 2004, however, the X.Org Server, a fork of XFree86, has become predominant.
While it is common to associate X with Unix, X servers also exist natively within other graphical environments. Hewlett-Packard's OpenVMS operating system includes a version of X with Common Desktop Environment (CDE), known as DECwindows, as its standard desktop environment. Apple's Mac OS X v10.3 (Panther) and v10.4 (Tiger) include X11.app, based on XFree86 4.3 and X11R6.6, with better Mac OS X integration. On Mac OS X v10.5 (Leopard), Apple's X11.app was based on X.org (X11R7.2 codebase) instead of XFree86 (currently at release X11R6.8). Third-party servers under Apple's older operating systems in the 1990s, System 7, and Mac OS 8 and 9, included Apple's MacX and White Pine Software's eXodus.
Microsoft Windows is not shipped with support for X, but many third-party implementations exist, as free and open source software such as Cygwin/X, WeirdX and Xming (free up to 188.8.131.52); freeware such as Mocha X Server; and proprietary products such as Xmanager, Exceed, eXcursion (by Hewlett-Packard), MKS X/Server, Reflection X, X-Win32 and Xming.
There are also Java implementations of X servers. WeirdX runs on any platform supporting Swing 1.1.*, and will run as an applet within most browsers. The Android X Server is an open source Java implementation that runs on Android devices.
When an operating system with a native windowing system hosts X in addition, the X system can either use its own normal desktop in a separate host window or it can run rootless, meaning the X desktop is hidden and the host windowing environment manages the geometry and appearance of the hosted X windows within the host screen.
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