History
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 It was shortly superseded by Version 11 (X11), however, in 1987.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. 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.

Architecture
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, GUI (graphical user interface) toolkits, and the like.

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 in X terminology it's reversed. 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.

Several different toolkits, window managers and desktop environments have been developed to provide consistency and improved services for X applications. Xaw (the Athena Widget Set), OLIT (OPEN LOOK Intrinsics Toolkit), XView, Motif, and Tk were among the earlier GUI toolkits for X. OLIT and XView are the base toolkits for AT&T and Sun Microsystems' OPEN LOOK GUI. Motif is the base toolkit for the Common Desktop Environment (CDE).

More recently, GTK+ (the GIMP Toolkit) and Qt have become the toolkits of choice for X hackers. Qt-based KDE (the K Desktop Environment, released from 1998 onwards) and GTK-based GNOME (the GNU Network Object Model Environment) are the latest additions to the X desktop, providing much greater application functionality and services than plain window managers or older desktop environments.

A very rudimentary X server on a piece of special-purpose or dedicated hardware is commonly referred to as an X terminal or "thin X client". These are popular for building inexpensive client parks for many users which may simultaneously use the same mainframe for heavy-duty tasks. This use is very much in line with the original intention of the MIT project.

X terminals explore the available hosts using an ad hoc network protocol called XDMCP to connect to a specific server which presents a list of available hosts. This server in turn may gather a list of available hosts using broadcast on the local network.

X Display Managers
The X Display Manager is used for keeping the X server process alive on the server, connecting it to a physical screen and serving a login prompt on this screen. The default display manager for X is XDM but sister projects have developed their own display managers:

- XDM
- GDM - developed by GNOME
- KDM - developed by KDE

Business model
The X Window System is distributed at no charge, with source code included 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.

Brands and Interoperability
While UNIX tends to standardize on X, there are also X servers for platforms with their own graphical environments, like Windows, MacOS, and OS/2.

The X variant most widely used on free UNIX-style systems is XFree86, which originally ran only on Intel x86-type PCs (hence the name), but now incorporates support for many more platforms.

X is named after an earlier window system called W (in the modern Roman alphabet the letter X comes right after W).

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "X Window System".