A window system for computers with bitmap graphical displays. Created at MIT in the 1980s, it is now under the supervision of X.Org (http://www.x.org/). Currently at version 11, release 6 (X11R6), the X Window System is more commonly called X11 or simply X. It is also often referred to as "X Windows", analogous to "Microsoft Windows", but this usage is incorrect and discouraged.
Although X provides functionality for drawing and moving Windows on the screen and also for providing a mouse cursor it provides none of the user interface features (such as buttons, menus, window title bars and so on) that people expect. These features are provided by other pieces of software, such as window managers, graphics toolkits, and the like.
Several different desktop environments have been developed to provide consistency and improved services for X Window applications. Motif and Openlook were the earliest. Motif was later replaced with CDE. KDE and Gnome are the latest additions, providing much greater application functionality and services than just simple window management.
X is based on a client-server model. A server program runs on a computer with a graphical display and communicates with various client programs, accepting requests for graphical output (windows) and sending back user input (keyboard, mouse). The communication protocol between server and client is network-transparent: the client programs can be run on the same machine as the server or equally well from other machines, possibly with different architectures and operating systems.
The client-server terminology is often confusing to new X users, because the terms are used differently than in other common contexts. In a typical X scenario, a user may be sitting at an X terminal or workstation where they interact with the keyboard and display, while their application program may be running on some big machine locked away in a computer room somewhere. Common terminology would refer to the workstation or terminal as a "client" and the other machine in the computer room as a "server", but the X terminology is different. The X terminology is used from the point of view of the application program, not the end user or the hardware. The application program is the client, which needs to use keyboard and display services. Therefore the workstation or terminal software is the X server, and the application program is the X client.
The X Window System is distributed at no charge, with source code and no restrictions on modification and redistribution. Due to the liberal licensing, a number of implementations (both free and proprietary) appeared that were based on the code from MIT. Originally developed for the Unix graphical workstations of the 1980s as part of MIT's Project Athena, these enhanced versions mainly added compatibility with specific operating systems and hardware. X became a part of the "standard" Unix offerings. Although other windowing systems for Unix exist, X is by far the most common.
X is named after an earlier window system called W (in the modern Roman alphabet the letter X comes right after W).
Debian X FAQ Entry on the X Window System
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In the words of its primary manual page, X(1), it is a "portable, network-transparent window system". Its primary distinction from other well-known window systems like Microsoft Windows and Apple's MacOS is that it was designed with the local area network in mind. You can run programs on one machine and display them on another.
Historically, the X Window System was initially conceived in 1984, at the Massachusetts Institute of Technology as a joint project between their Laboratory for Computer Science and the Digital Equipment Corporation. The initial impetus for the X Window System was MIT's Project Athena, which sought to provide easy access to computing resources for all students; because MIT could not buy all the workstations needed, nor was any single vendor willing to donate them, a platform-independent graphics system was required. The first version of the X Window System to be widely deployed was Version 10 (X10). It was shortly superseded by Version 11 (X11), however, in 1987.
In 1988, a non-profit group called the (MIT) X Consortium was formed to direct future development of X standards in an atmosphere inclusive of many commercial and educational interests. The X Consortium produced several significant revisions to X11, concluding with Release 6 in 1994 (X11R6).
The X Consortium dissolved at the end of 1996, producing a final, small revision to X11R6 called X11R6.3. Ownership of X then passed to the Open Group, an outgrowth of the Open Software Foundation (OSF), who produced the popular Motif widget set for X. In early 1998, the Open Group released a further revision to X11R6, called X11R6.4 -- a departure from the traditional licensing terms, however, prevented adoption of this version of the X Window System by many vendors, including the XFree86 Project, Inc. (see below). In late 1998, the Open Group relicensed X11R6.4 under terms identical with the traditional license.
In May 1999, stewardship of the X Window System passed from the Open Group to X.Org, a non-profit organization focused exclusively on maintenance and further development of the X Window System. X.Org has supervised the release of X11R6.5.1.