X11

X11, formally the X Window System version 11, is a windowing system for bitmap displays that provides network transparency and a framework for graphical user interfaces on Unix-like and other operating systems. Introduced in 1987 by the MIT X Consortium, it established a standardized protocol and set of libraries for building graphical applications.

The system operates on a client–server model. The X server runs on the display hardware and manages input devices and windows, while X clients are the applications that request drawing operations and event handling from the server. The server handles window management, drawing primitives, and input focus, and a separate window manager often provides window decorations and placement. Communication between clients and the server follows the X11 protocol, which is asynchronous and typed through requests, events, and errors. Common client libraries include Xlib and XCB.

Extensions expand X11 capabilities. Notable examples are RandR for dynamic display configuration, XInput for advanced input, XRender for modern rendering, and Composite and Damage for compositing and off-screen rendering. Security relies on access control mechanisms such as MIT-MAGIC-COOKIE-1 and xauth, enabling both local and remote display usage over networks.

History and status. The X Window System evolved under the MIT X Consortium and is currently maintained by the X.Org Foundation and a broad community. While it remains widely deployed, many modern Linux and Unix desktop environments are adopting Wayland as a newer display server protocol, with X11 continuing to provide compatibility and a large ecosystem of applications and toolkits.

Typical environments. X11 runs on many Unix-like systems, BSDs, and macOS via ports like XQuartz; Windows users often access X11 apps through external servers such as Xming or Cygwin/X.