The X Window System (X11, or simply X) is a windowing system for

bitmap In computing, a bitmap (also called raster) graphic is an image formed from rows of different colored pixels. A GIF is an example of a graphics image file that uses a bitmap. As a noun, the term "bitmap" is very often used to refer to a partic ...

displays, common on

Unix-like A Unix-like (sometimes referred to as UN*X, *nix or *NIX) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Uni ...

operating systems. X originated as part of

Project Athena Project Athena was a joint project of MIT, Digital Equipment Corporation, and IBM to produce a campus-wide distributed computing environment for educational use. It was launched in 1983, and research and development ran until June 30, 1991. , At ...

Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a Private university, private research university in Cambridge, Massachusetts, United States. Established in 1861, MIT has played a significant role in the development of many areas of moder ...

(MIT) in 1984. The X protocol has been at version 11 (hence "X11") since September 1987. The X.Org Foundation leads the X project, with the current reference implementation,

X.Org Server X.Org Server is the free and open-source implementation of the X Window System (X11) display server stewarded by the X.Org Foundation. Implementations of the client-side X Window System protocol exist in the form of ''X11 libraries'', which ...

, available as

free and open-source software Free and open-source software (FOSS) is software available under a license that grants users the right to use, modify, and distribute the software modified or not to everyone free of charge. FOSS is an inclusive umbrella term encompassing free ...

under the

MIT License The MIT License is a permissive software license originating at the Massachusetts Institute of Technology (MIT) in the late 1980s. As a permissive license, it puts very few restrictions on reuse and therefore has high license compatibility. Unl ...

and similar permissive licenses.

Purpose and abilities

X is an architecture-independent system for remote graphical user interfaces and input device capabilities. Each person using a networked terminal has the ability to interact with the display with any type of user input device. In its standard distribution it is a complete, albeit simple, display and interface solution which delivers a standard toolkit and

protocol stack The protocol stack or network stack is an implementation of a computer networking protocol suite or protocol family. Some of these terms are used interchangeably but strictly speaking, the ''suite'' is the definition of the communication protoc ...

for building graphical user interfaces on most

operating systems and

OpenVMS OpenVMS, often referred to as just VMS, is a multi-user, multiprocessing and virtual memory-based operating system. It is designed to support time-sharing, batch processing, transaction processing and workstation applications. Customers using Op ...

, and has been ported to many other contemporary general purpose

operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

s. X provides the basic framework, or primitives, for building such GUI environments: drawing and moving

windows Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...

on the display and interacting with a mouse, keyboard or touchscreen. X does not mandate the

user interface In the industrial design field of human–computer interaction, a user interface (UI) is the space where interactions between humans and machines occur. The goal of this interaction is to allow effective operation and control of the machine fro ...

; individual client programs handle this. Programs may use X's graphical abilities with no user interface. As such, the visual styling of X-based environments varies greatly; different programs may present radically different interfaces. Unlike most earlier display protocols, X was specifically designed to be used over network connections rather than on an integral or attached display device. X features

network transparency Network transparency refers to the ability of a protocol to transmit data over the network in a manner which is not observable to those using the applications that are using the protocol. In this way, users of a particular application may access ...

, which means an X program running on a computer somewhere on a network (such as the Internet) can display its user interface on an X server running on some other computer on the network. The X server is typically the provider of graphics resources and keyboard/mouse events to X clients, meaning that the X server is usually running on the computer in front of a human user, while the X client applications run anywhere on the network and communicate with the user's computer to request the rendering of graphics content and receive events from input devices including keyboards and mice. The fact that the term "server" is applied to the software in front of the user is often surprising to users accustomed to their programs being clients to services on remote computers. Here, rather than a remote database being the resource for a local app, the user's graphic display and input devices become resources made available by the local X server to both local and remotely hosted X client programs who need to share the user's graphics and input devices to communicate with the user. X's network protocol is based on X command primitives. This approach allows both 2D and (through extensions like GLX) 3D operations by an X client application which might be running on a different computer to still be fully accelerated on the X server's display. For example, in classic OpenGL (before version 3.0), display lists containing large numbers of objects could be constructed and stored entirely in the X server by a remote X client program, and each then rendered by sending a single glCallList(which) across the network. X provides no native support for audio; several projects exist to fill this niche, some also providing transparent network support.

Software architecture

X uses a client–server model: an X server communicates with various ''client'' programs. The server accepts requests for graphical output (windows) and sends back user input (from keyboard, mouse, or touchscreen). The server may function as: * an application displaying to a window of another display system * a system program controlling the video output of a PC * a dedicated piece of hardware This client–server terminologythe user's terminal being the server and the applications being the clientsoften confuses new X users, because the terms appear reversed. But X takes the perspective of the application, rather than that of the end-user: X provides display and I/O services to applications, so it is a server; applications use these services, thus they are clients. The

communication protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any variation of a physical quantity. The protocol defines the rules, syntax, semantics (computer science), sem ...

between server and client operates network-transparently: the client and server may run on the same machine or on different ones, possibly with different architectures and operating systems. A client and server can even communicate securely over the Internet by tunneling the connection over an encrypted network session. An X client itself may emulate an X server by providing display services to other clients. This is known as "X nesting". Open-source clients such as Xnest and

Xephyr Xephyr is a display server software implementing the X11 display server protocol based on KDrive which targets a window on a host X Server as its framebuffer. It is written by Matthew Allum. Xephyr is an X-on-X implementation and runs on X.O ...

support such X nesting.

Remote desktop

To run an X client application on a remote machine, the user may do the following: * on the local machine, open a terminal window * use command to connect to the remote machine * request a local display/input service (e.g., export DISPLAY='' ser's machine':0 if not using SSH with X forwarding enabled) The remote X client application will then make a connection to the user's local X server, providing display and input to the user. Alternatively, the local machine may run a small program that connects to the remote machine and starts the client application. Practical examples of remote clients include: * administering a remote machine graphically (similar to using remote desktop, but with single windows) * using a client application to join with large numbers of other terminal users in collaborative workgroups * running a computationally intensive simulation on a remote machine and displaying the results on a local desktop machine * running graphical software on several machines at once, controlled by a single display, keyboard and mouse

User interfaces

X primarily defines protocol and graphics primitivesit deliberately contains no specification for application user-interface design, such as button, menu, or window title-bar styles. Instead, application softwaresuch as window managers, GUI widget toolkits and desktop environments, or application-specific graphical user interfacesdefine and provide such details. As a result, there is no ''typical'' X interface and several different desktop environments have become popular among users. A window manager controls the placement and appearance of application windows. This may result in desktop interfaces reminiscent of those of Microsoft Windows or of the Apple Macintosh (examples include GNOME 2, KDE Plasma, Xfce) or have radically different controls (such as a tiling window manager, like wmii or Ratpoison). Some interfaces such as

Sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecul ...

ChromeOS ChromeOS, sometimes styled as chromeOS and formerly styled as Chrome OS, is an operating system designed and developed by Google. It is derived from the open-source operating system and uses the Google Chrome web browser as its principal user ...

eschew the desktop metaphor altogether, simplifying their interfaces for specialized applications. Window managers range in sophistication and complexity from the bare-bones (''e.g.'', twm, the basic window manager supplied with X, or evilwm, an extremely light window manager) to the more comprehensive desktop environments such as Enlightenment and even to application-specific window managers for vertical markets such as point-of-sale. Many users use X with a desktop environment, which, aside from the window manager, includes various applications using a consistent user interface. Popular desktop environments include

GNOME A gnome () is a mythological creature and diminutive spirit in Renaissance magic and alchemy, introduced by Paracelsus in the 16th century and widely adopted by authors, including those of modern fantasy literature. They are typically depict ...

KDE Plasma KDE Plasma is a Shell (computing), graphical shell developed by the KDE community for Unix-like operating systems. It serves as the interface layer between the user and the operating system, providing a graphical user interface (GUI) and workspa ...

and

Xfce Xfce or XFCE (pronounced as four individual letters, ) is a Free and open-source software, free and open-source desktop environment for Linux and other Unix-like operating systems. Xfce aims to be fast and Lightweight software, lightweight whil ...

. The UNIX 98 standard environment is the

Common Desktop Environment The Common Desktop Environment (CDE) is a desktop environment for Unix and OpenVMS, based on the Motif (software), Motif widget toolkit. It was part of the UNIX 98, UNIX 98 Workstation Product Standard, and was for a long time the Unix desktop a ...

(CDE). The freedesktop.org initiative addresses interoperability between desktops and the components needed for a competitive X desktop.

Implementations

The X.Org implementation is the canonical implementation of X. Owing to liberal licensing, a number of variations, both free and open source and proprietary, have appeared. Commercial Unix vendors have tended to take the reference implementation and adapt it for their hardware, usually customizing it and adding proprietary extensions. Until 2004, XFree86 provided the most common X variant on free Unix-like systems. XFree86 started as a port of X to 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. VMS Software Inc.'s OpenVMS operating system includes a version of X with Common Desktop Environment (CDE), known as DECwindows, as its standard desktop environment. Apple originally ported X to macOS in the form of X11.app, but that has been deprecated in favor of the XQuartz implementation. Third-party servers under Apple's older operating systems in the 1990s, System 7, and

Mac OS Mac operating systems were developed by Apple Inc. in a succession of two major series. In 1984, Apple debuted the operating system that is now known as the classic Mac OS with its release of the original Macintosh System Software. The system ...

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, and proprietary products such as Exceed, 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.

X terminals

An ''X terminal'' is a thin client that only runs an X server. This architecture became popular for building inexpensive terminal parks for many users to simultaneously use the same large computer server to execute application programs as clients of each user's X terminal. This use is very much aligned with the original intention of the MIT project. X terminals explore the network (the local

broadcast domain A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments. In te ...

) using the

X Display Manager Control Protocol In the X Window System, an X display manager is a graphical login manager which starts a login session on an X Window System, X server from the same or another computer. A display manager presents the user with a login, login screen. A sessio ...

to generate a list of available hosts that are allowed as clients. One of the client hosts should run an X display manager. A limitation of X terminals and most thin clients is that they are not capable of any input or output other than the keyboard, mouse, and display. All relevant data is assumed to exist solely on the remote server, and the X terminal user has no methods available to save or load data from a local

peripheral A peripheral device, or simply peripheral, is an auxiliary hardware device that a computer uses to transfer information externally. A peripheral is a hardware component that is accessible to and controlled by a computer but is not a core compo ...

device. Dedicated (hardware) X terminals have fallen out of use; a PC or modern

thin client In computer networking, a thin client, sometimes called slim client or lean client, is a simple (low-Computer performance, performance) computer that has been Program optimization, optimized for Remote desktop, establishing a remote connectio ...

with an X server typically provides the same functionality at the same, or lower, cost.

Limitations and criticism

'' The Unix-Haters Handbook'' (1994) devoted a full chapter to the problems of X. ''Why X Is Not Our Ideal Window System'' (1990) by Gajewska, Manasse and McCormack detailed problems in the protocol with recommendations for improvement.

User interface issues

The lack of design guidelines in X has resulted in several vastly different interfaces, and in applications that have not always worked well together. The

Inter-Client Communication Conventions Manual In computing, the Inter-Client Communication Conventions Manual (ICCCM or I39L short for "I", 39 letters and "L")Motif and CDE did not alleviate problems. This has frustrated users and programmers. Graphics programmers now generally address consistency of application

look and feel In software design, the look and feel of a graphical user interface comprises aspects of its design, including elements such as colors, shapes, layout, and typefaces (the "look"), as well as the behavior of dynamic elements such as buttons, boxes ...

and communication by coding to a specific desktop environment or to a specific widget toolkit, which also avoids having to deal directly with the ICCCM. X also lacks native support for user-defined stored procedures on the X server, in the manner of

NeWS News is information about current events. This may be provided through many different Media (communication), media: word of mouth, printing, Mail, postal systems, broadcasting, Telecommunications, electronic communication, or through the te ...

there is no

Turing-complete In computability theory, a system of data-manipulation rules (such as a model of computation, a computer's instruction set, a programming language, or a cellular automaton) is said to be Turing-complete or computationally universal if it can be ...

scripting facility. Various desktop environments may thus offer their own (usually mutually incompatible) facilities.

Computer accessibility related issues

Systems built upon X may have accessibility issues that make utilization of a computer difficult for disabled users, including

right click A context menu (also called contextual, shortcut, and pop up or pop-up menu) is a Menu (computing), menu in a graphical user interface (GUI) that appears upon user interaction, such as a right-click Mouse (computing)#Operation, mouse operation. ...

double click A double-click is the act of pressing a computer mouse button twice quickly without moving the mouse. Double-clicking allows two different actions to be associated with the same mouse button. It was developed by Tim Mott of Xerox Palo Alto Resear ...

, middle click, mouse-over, and focus stealing. Some X11 clients deal with accessibility issues better than others, so persons with accessibility problems are not locked out of using X11. However, there is no accessibility standard or accessibility guidelines for X11. Within the X11 standards process there is no working group on accessibility; however, accessibility needs are being addressed by software projects to provide these features on top of X. The

Orca The orca (''Orcinus orca''), or killer whale, is a toothed whale and the largest member of the oceanic dolphin family. The only extant species in the genus '' Orcinus'', it is recognizable by its black-and-white-patterned body. A cosmopol ...

project adds accessibility support to the X Window System, including implementing an API ( AT-SPI). This is coupled with GNOME's ATK to allow for accessibility features to be implemented in X programs using the GNOME/GTK APIs. KDE provides a different set of accessibility software, including a text-to-speech converter and a screen magnifier. The other major desktops (LXDE, Xfce and Enlightenment) attempt to be compatible with ATK.

Network

An X client cannot generally be detached from one server and reattached to another unless its code specifically provides for it (

Emacs Emacs (), originally named EMACS (an acronym for "Editor Macros"), is a family of text editors that are characterized by their extensibility. The manual for the most widely used variant, GNU Emacs, describes it as "the extensible, customizable, s ...

is one of the few common programs with this ability). As such, moving an entire session from one X server to another is generally not possible. However, approaches like

Virtual Network Computing VNC (Virtual Network Computing) is a graphical desktop-sharing system that uses the RFB protocol, Remote Frame Buffer protocol (RFB) to remotely control another computer. It transmits the computer keyboard, keyboard and computer mouse, mouse in ...

(VNC), NX and Xpra allow a virtual session to be reached from different X servers (in a manner similar to GNU Screen in relation to terminals), and other applications and toolkits provide related facilities. Workarounds like x11vnc (''VNC :0 viewers''), Xpra's shadow mode and NX's nxagent shadow mode also exist to make the current X-server screen available. This ability allows the user interface (mouse, keyboard, monitor) of a running application to be switched from one location to another without stopping and restarting the application. Network traffic between an X server and remote X clients is not encrypted by default. An attacker with a

packet sniffer A packet analyzer (also packet sniffer or network analyzer) is a computer program or computer hardware such as a packet capture appliance that can Traffic analysis, analyze and Logging (computing), log traffic that passes over a computer netwo ...

can intercept it, making it possible to view anything displayed to or sent from the user's screen. The most common way to encrypt X traffic is to establish a

Secure Shell The Secure Shell Protocol (SSH Protocol) is a cryptographic network protocol for operating network services securely over an unsecured network. Its most notable applications are remote login and command-line execution. SSH was designed for ...

(SSH) tunnel for communication. Like all

s, when using X across a network,

bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...

limitations can impede the use of

-intensive applications that require rapidly updating large portions of the screen with low latency, such as 3D animation or photo editing. Even a relatively small ''uncompressed'' 640×480×24 bit 30 fps video stream (~211 Mbit/s) can easily outstrip the bandwidth of a 100 Mbit/s network for a single client. In contrast, modern versions of X generally have extensions such as

Mesa A mesa is an isolated, flat-topped elevation, ridge, or hill, bounded from all sides by steep escarpments and standing distinctly above a surrounding plain. Mesas consist of flat-lying soft sedimentary rocks, such as shales, capped by a ...

allowing local display of a local program's graphics to be optimized to bypass the network model and directly control the video card, for use with full-screen video, rendered 3D applications, and other such applications.

Client–server separation

X's design requires the clients and server to operate separately, and device independence and the separation of client and server incur overhead. Most of the overhead comes from network

round-trip delay time In telecommunications, round-trip delay (RTD) or round-trip time (RTT) is the amount of time it takes for a signal to be sent ''plus'' the amount of time it takes for acknowledgement of that signal having been received. This time delay includes p ...

between client and server ( latency) rather than from the protocol itself: the best solutions to performance issues depend on efficient application design. A common criticism of X is that its network features result in excessive complexity and decreased performance if only used locally. Modern X implementations use

Unix domain socket A Unix domain socket (UDS), a.k.a. local socket, a.k.a. inter-process communication (IPC) socket, is a communication endpoint for exchanging data between processes executing in the same Unix or Unix-like operating system. The name, ''Unix domain ...

s for efficient connections on the same host. Additionally shared memory (via the

MIT-SHM The MIT Shared Memory Extension or MIT-SHM or XShm is an X Window System extension for exchange of image data between client and server using shared memory (usually ). The mechanism only works when both pieces are on the same computer. The basi ...

extension) can be employed for faster client–server communication. However, the programmer must still explicitly activate and use the shared memory extension. It is also necessary to provide fallback paths in order to stay compatible with older implementations, and in order to communicate with non-local X servers.

Competitors

Some people have attempted writing alternatives to and replacements for X. Historical alternatives include

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

and

NeXT NeXT, Inc. (later NeXT Computer, Inc. and NeXT Software, Inc.) was an American technology company headquartered in Redwood City, California that specialized in computer workstations for higher education and business markets, and later develope ...

Display PostScript Display PostScript (or DPS) is a 2D graphics engine system for computers that uses the PostScript (PS) imaging model and language to generate on-screen graphics. PS was originally developed for computer printing, to which DPS adds a number of fe ...

, both

PostScript PostScript (PS) is a page description language and dynamically typed, stack-based programming language. It is most commonly used in the electronic publishing and desktop publishing realm, but as a Turing complete programming language, it c ...

-based systems supporting user-definable display-side procedures, which X lacked. Current alternatives include: *

macOS macOS, previously OS X and originally Mac OS X, is a Unix, Unix-based operating system developed and marketed by Apple Inc., Apple since 2001. It is the current operating system for Apple's Mac (computer), Mac computers. With ...

(and its mobile counterpart,

iOS Ios, Io or Nio (, ; ; locally Nios, Νιός) is a Greek island in the Cyclades group in the Aegean Sea. Ios is a hilly island with cliffs down to the sea on most sides. It is situated halfway between Naxos and Santorini. It is about long an ...

) implements its windowing system, which is known as

Quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

. When

Apple Computer Apple Inc. is an American multinational corporation and technology company headquartered in Cupertino, California, in Silicon Valley. It is best known for its consumer electronics, software, and services. Founded in 1976 as Apple Computer Co ...

bought NeXT, and used

NeXTSTEP NeXTSTEP is a discontinued object-oriented, multitasking operating system based on the Mach kernel and the UNIX-derived BSD. It was developed by NeXT, founded by Steve Jobs, in the late 1980s and early 1990s and was initially used for its ...

to construct Mac OS X, it replaced Display PostScript with Quartz. Mike Paquette, one of the authors of Quartz, explained that if Apple had added support for all the features it wanted to include into X11, it would not bear much resemblance to X11 nor be compatible with other servers anyway. * Wayland is being developed by several X.Org developers as a prospective replacement for X. It works directly with the GPU hardware, via DRI. Wayland can run an X server as a Wayland compositor, which can be rootless. The project reached version 1.0 in 2012. Like Android, Wayland is EGL-based. *

Mir ''Mir'' (, ; ) was a space station operated in low Earth orbit from 1986 to 2001, first by the Soviet Union and later by the Russia, Russian Federation. ''Mir'' was the first modular space station and was assembled in orbit from 1986 to ...

was a project from Canonical Ltd. with goals similar to Wayland. Mir was intended to work with mobile devices using ARM chipsets (a stated goal was compatibility with Android device-drivers) as well as x86 desktops. Like Android, Mir/UnityNext were EGL-based. Backwards compatibility with X client-applications was accomplished via Xmir. The project has since moved to being a

Wayland compositor Wayland is a communication protocol that specifies the communication between a display server and its clients, as well as a C library implementation of that protocol. A display server using the Wayland protocol is called a '' Wayland composito ...

instead of being an alternative display server. * Other alternatives attempt to avoid the overhead of X by working directly with the hardware; such projects include DirectFB. The

Direct Rendering Infrastructure The Direct Rendering Infrastructure (DRI) is the framework comprising the modern Linux graphics stack which allows unprivileged user-space programs to issue commands to graphics hardware without conflicting with other programs. The main use ...

(DRI) provides a kernel-level interface to the

framebuffer A framebuffer (frame buffer, or sometimes framestore) is a portion of random-access memory (RAM) containing a bitmap that drives a video display. It is a memory buffer containing data representing all the pixels in a complete video frame. Mode ...

. Additional ways to achieve a functional form of the "network transparency" feature of X, via network transmissibility of graphical services, include: *

(VNC), a very low-level system which sends compressed bitmaps across the network; the Unix implementation includes an X server *

Remote Desktop Protocol Remote Desktop Protocol (RDP) is a proprietary protocol developed by Microsoft Corporation which provides a user with a graphical interface to connect to another computer over a network connection. The user employs RDP client software for this pu ...

(RDP), which is similar to VNC in purpose, but originated on Microsoft Windows before being ported to Unix-like systems, ''e.g.'' NX * Citrix XenApp, an X-like protocol and application stack for Microsoft Windows *

Tarantella Tarantella () is a group of various Southern Italy, southern Italian Italian folk dance, folk dances originating in the regions of Calabria, Campania, Sicilia, and Apulia. It is characterized by a fast Beat (music), upbeat tempo, usually in Ti ...

, which provides a Java-based remote-gui-client for use in web browsers

History

Predecessors

Several bitmap display systems preceded X. From

Xerox Xerox Holdings Corporation (, ) is an American corporation that sells print and electronic document, digital document products and services in more than 160 countries. Xerox was the pioneer of the photocopier market, beginning with the introduc ...

came the

Alto The musical term alto, meaning "high" in Italian (Latin: '' altus''), historically refers to the contrapuntal part higher than the tenor and its associated vocal range. In four-part voice leading alto is the second-highest part, sung in ch ...

(1973) and the

Star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

(1981). From

Apollo Computer Apollo Computer Inc. was an American technology corporation headquartered and founded in Chelmsford, Massachusetts. It was founded in 1980 by William Poduska (a founder of Prime Computer) and others. Apollo Computer developed and produced Apoll ...

came Display Manager (1981). From

Apple An apple is a round, edible fruit produced by an apple tree (''Malus'' spp.). Fruit trees of the orchard or domestic apple (''Malus domestica''), the most widely grown in the genus, are agriculture, cultivated worldwide. The tree originated ...

came the

Lisa Lisa or LISA may refer to: People People with the mononym * Lisa (Japanese musician, born 1974), stylized "LISA" * Lisa, stagename of Japanese singer Lisa Komine (born 1978) * Lisa (South Korean singer) (born 1980) * Lisa (Japanese musician, b ...

(1983) and the

Macintosh Mac is a brand of personal computers designed and marketed by Apple Inc., Apple since 1984. The name is short for Macintosh (its official name until 1999), a reference to the McIntosh (apple), McIntosh apple. The current product lineup inclu ...

(1984). The

Unix Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...

world had the

Andrew Project The Andrew Project was a distributed computing environment developed at Carnegie Mellon University beginning in 1982. It was an ambitious project for its time and resulted in an unprecedentedly vast and accessible university computing infrastructu ...

(1982) and

Rob Pike Robert Pike (born 1956) is a Canadian programmer and author. He is best known for his work on the Go programming language while working at Google and the Plan 9 operating system while working at Bell Labs, where he was a member of the Unix t ...

's Blit terminal (1982). Carnegie Mellon University produced a remote-access application called Alto Terminal, that displayed overlapping windows on the Xerox Alto, and made remote hosts (typically DEC VAX systems running Unix) responsible for handling window-exposure events and refreshing window contents as necessary. X derives its name as a successor to a pre-1983 window system called W (the letter preceding X in the

English alphabet Modern English is written with a Latin-script alphabet consisting of 26 Letter (alphabet), letters, with each having both uppercase and lowercase forms. The word ''alphabet'' is a Compound (linguistics), compound of ''alpha'' and ''beta'', t ...

). W ran under the V operating system. W used a network protocol supporting terminal and graphics windows, the server maintaining display lists.

Origin and early development

The original idea of X emerged at MIT in 1984 as a collaboration between

Jim Gettys Jim Gettys (born 15 October 1953) is an American computer programmer. Activity Gettys worked at Digital Equipment Corporation, DEC's Cambridge Research Laboratory. He is one of the original developers of the X Window System at MIT and worked ...

(of

) and

Bob Scheifler Robert William Scheifler (born June 24, 1954) is an American computer scientist. He was born in Kirkwood, Missouri. He is most notable for leading the development of the X Window System from the project's inception in 1984 until the closure of th ...

(of the MIT Laboratory for Computer Science). Scheifler needed a usable display environment for debugging the Argus system.

(a joint project between DEC, MIT and

IBM International Business Machines Corporation (using the trademark IBM), nicknamed Big Blue, is an American Multinational corporation, multinational technology company headquartered in Armonk, New York, and present in over 175 countries. It is ...

to provide easy access to computing resources for all students) needed a platform-independent graphics system to link together its heterogeneous multiple-vendor systems; the window system then under development in

Carnegie Mellon University Carnegie Mellon University (CMU) is a private research university in Pittsburgh, Pennsylvania, United States. The institution was established in 1900 by Andrew Carnegie as the Carnegie Technical Schools. In 1912, it became the Carnegie Institu ...

did not make licenses available, and no alternatives existed. The project solved this by creating a protocol that could both run local applications and call on remote resources. In mid-1983 an initial port of W to Unix ran at one-fifth of its speed under V; in May 1984, Scheifler replaced the

synchronous Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchrono ...

protocol of W with an

asynchronous Asynchrony is any dynamic far from synchronization. If and as parts of an asynchronous system become more synchronized, those parts or even the whole system can be said to be in sync. Asynchrony or asynchronous may refer to: Electronics and com ...

protocol and the display lists with immediate mode graphics to make X version 1. X became the first windowing system environment to offer true hardware independence and vendor independence. Scheifler, Gettys and Ron Newman set to work and X progressed rapidly. They released Version 6 in January 1985. DEC, then preparing to release its first

Ultrix Ultrix (officially all-caps ULTRIX) is the brand name of Digital Equipment Corporation's (DEC) discontinued native Unix operating systems for the PDP-11, VAX, MicroVAX and DECstations. History The initial development of Unix occurred on DEC eq ...

workstation, judged X the only windowing system likely to become available in time. DEC engineers ported X6 to DEC's QVSS display on

MicroVAX The MicroVAX is a discontinued family of low-cost minicomputers developed and manufactured by Digital Equipment Corporation (DEC). The first model, the MicroVAX I, shipped in 1984. The series uses processors that implement the VAX instruction se ...

. In the second quarter of 1985, X acquired

color Color (or colour in English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is the visual perception based on the electromagnetic spectrum. Though co ...

support to function in the DEC

VAXstation The VAXstation is a discontinued family of workstation computers developed and manufactured by Digital Equipment Corporation using processors implementing the VAX instruction set architecture. VAXstation systems were typically shipped with eithe ...

-II/GPX, forming what became version 9. A group at

Brown University Brown University is a Private university, private Ivy League research university in Providence, Rhode Island, United States. It is the List of colonial colleges, seventh-oldest institution of higher education in the US, founded in 1764 as the ' ...

ported version 9 to the

IBM RT PC The IBM RT PC (RISC Technology Personal Computer) is a family of workstation computers from IBM introduced in 1986. These were the first commercial computers from IBM that were based on a reduced instruction set computer (RISC) architecture. Th ...

, but problems with reading unaligned data on the RT forced an incompatible protocol change, leading to version 10 in late 1985. X10R1 was released in 1985. By 1986, outside organizations had begun asking for X. X10R2 was released in January 1986, then X10R3 in February 1986. Although MIT had licensed X6 to some outside groups for a fee, it decided at this time to license X10R3 and future versions under what became known as the

, intending to popularize X further and, in return, hoping that many more applications would become available. X10R3 became the first version to achieve wide deployment, with both DEC and

Hewlett-Packard The Hewlett-Packard Company, commonly shortened to Hewlett-Packard ( ) or HP, was an American multinational information technology company. It was founded by Bill Hewlett and David Packard in 1939 in a one-car garage in Palo Alto, California ...

releasing products based on it. Other groups ported X10 to

Apollo Apollo is one of the Twelve Olympians, Olympian deities in Ancient Greek religion, ancient Greek and Ancient Roman religion, Roman religion and Greek mythology, Greek and Roman mythology. Apollo has been recognized as a god of archery, mu ...

and to

workstations and even to the IBM

PC/AT The IBM Personal Computer AT (model 5170, abbreviated as IBM AT or PC/AT) was released in 1984 as the fourth model in the IBM Personal Computer line, following the IBM PC/XT and its IBM Portable PC variant. It was designed around the Intel 802 ...

. Demonstrations of the first commercial application for X (a mechanical computer-aided engineering system from Cognition Inc. that ran on VAXes and remotely displayed on PCs running an X server ported by Jim Fulton and Jan Hardenbergh) took place at the Autofact trade show at that time. The last version of X10, X10R4, appeared in December 1986. Attempts were made to enable X servers as real-time collaboration devices, much as

(VNC) would later allow a desktop to be shared. One such early effort was Philip J. Gust's SharedX tool. Although X10 offered interesting and powerful functionality, it had become obvious that the X protocol could use a more hardware-neutral redesign before it became too widely deployed, but MIT alone would not have the resources available for such a complete redesign. As it happened, DEC's Western Software Laboratory found itself between projects with an experienced team. Smokey Wallace of DEC WSL and Jim Gettys proposed that DEC WSL build X11 and make it freely available under the same terms as X9 and X10. This process started in May 1986, with the protocol finalized in August. Alpha testing of the software started in February 1987, beta-testing in May; the release of X11 finally occurred on 15 September 1987. The X11 protocol design, led by Scheifler, was extensively discussed on open mailing lists on the nascent Internet that were bridged to USENET newsgroups. Gettys moved to California to help lead the X11 development work at WSL from DEC's Systems Research Center, where Phil Karlton and Susan Angebrandt led the X11 sample server design and implementation. X therefore represents one of the first very large-scale distributed

free and open source software Free and open-source software (FOSS) is software available under a Software license, license that grants users the right to use, modify, and distribute the software modified or not to everyone free of charge. FOSS is an inclusive umbrella term ...

projects.

The MIT X Consortium and the X Consortium, Inc.

By the late 1980s X was, Simson Garfinkel wrote in 1989, "Athena's most important single achievement to date". DEC reportedly believed that its development alone had made the company's donation to MIT worthwhile. Gettys joined the design team for the VAXstation 2000 to ensure that X—which DEC called DECwindows—would run on it, and the company assigned 1,200 employees to port X to both Ultrix and VMS. By 1990 IBM and

Motorola Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois. It was founded by brothers Paul and Joseph Galvin in 1928 and had been named Motorola since 1947. Many of Motorola's products had been ...

announced their own X terminals.

Bill Joy William Nelson Joy (born November 8, 1954) is an American computer engineer and venture capitalist. He co-founded Sun Microsystems in 1982 along with Scott McNealy, Vinod Khosla, and Andy Bechtolsheim, and served as Chief Scientist and CTO ...

Sun Microsystems Sun Microsystems, Inc., often known as Sun for short, was an American technology company that existed from 1982 to 2010 which developed and sold computers, computer components, software, and information technology services. Sun contributed sig ...

—which made

diskless workstations A diskless node (or diskless workstation) is a workstation or personal computer without disk drives, which employs network booting to load its operating system from a server (computing), server. (A computer may also be said to ''act as a diskless ...

competing with X terminals—argued that X was technically flawed and could overwhelm networks. In 1987, with the success of X11 becoming apparent, MIT wished to relinquish the stewardship of X, but at a June 1987 meeting with nine vendors, the vendors told MIT that they believed in the need for a neutral party to keep X from fragmenting in the marketplace. In January 1988, the ''MIT X Consortium'' formed as a non-profit vendor group, with Scheifler as director, to direct the future development of X in a neutral atmosphere inclusive of commercial and educational interests. Jim Fulton joined in January 1988 and Keith Packard in March 1988 as senior developers, with Jim focusing on

Xlib Xlib (also known as libX11) is an X Window System protocol client library (computer science), library written in the C (programming language), C programming language. It contains subroutine, functions for interacting with an X Server (computi ...

fonts In movable type, metal typesetting, a font is a particular #Characteristics, size, weight and style of a ''typeface'', defined as the set of fonts that share an overall design. For instance, the typeface Bauer Bodoni (shown in the figure) inclu ...

, window managers, and utilities; and Keith re-implementing the server. Donna Converse, Chris D. Peterson, and Stephen Gildea joined later that year, focusing on toolkits and widget sets, working closely with Ralph Swick of MIT Project Athena. The MIT X Consortium produced several significant revisions to X11, the first (Release 2X11R2) in February 1988. Jay Hersh joined the staff in January 1991 to work on the PEX and X113D functionality. He was followed soon after by Ralph Mor (who also worked on PEX) and Dave Sternlicht. In 1993, as the MIT X Consortium prepared to depart from MIT, the staff were joined by R. Gary Cutbill, Kaleb Keithley, and David Wiggins.Robert W. Scheifler and James Gettys: X Window System: Core and extension protocols: X version 11, releases 6 and 6.1, Digital Press 1996, CDE Application Builder

In 1993, the X Consortium, Inc. (a non-profit corporation) formed as the successor to the MIT X Consortium. It released X11R6 on 16 May 1994. In 1995 it took on the development of the Motif toolkit and of the

for Unix systems. The X Consortium dissolved at the end of 1996, producing a final revision, X11R6.3, and a legacy of increasing commercial influence in the development.

The Open Group

In January 1997, the X Consortium passed stewardship of X to

The Open Group The Open Group is a global consortium that seeks to "enable the achievement of business objectives" by developing " open, vendor-neutral technology standards and certifications." It has 900+ member organizations and provides a number of services ...

, a vendor group formed in early 1996 by the merger of the

Open Software Foundation The Open Software Foundation, Inc. (OSF), was a not-for-profit industry consortium for creating an open standard for an implementation of the operating system Unix. It was formed in 1988 and merged with X/Open in 1996, to become The Open Group. ...

and

X/Open X/Open group (also known as the Open Group for Unix Systems and incorporated in 1987 as X/Open Company, Ltd.) was a consortium founded by several European UNIX systems manufacturers in 1984 to identify and promote open standards in the field of info ...

. The Open Group released X11R6.4 in early 1998. Controversially, X11R6.4 departed from the traditional liberal licensing terms, as the Open Group sought to assure funding for the development of X, and specifically cited

XFree86 XFree86 is an implementation of the X Window System. It was originally written for Unix-like operating systems on IBM PC compatibles and was available for many other operating systems and platforms. It is free software, free and Open-source softw ...

as not significantly contributing to X. The new terms would have made X no longer

free software Free software, libre software, libreware sometimes known as freedom-respecting software is computer software distributed open-source license, under terms that allow users to run the software for any purpose as well as to study, change, distribut ...

: zero-cost for noncommercial use, but a fee otherwise. After XFree86 seemed poised to

fork In cutlery or kitchenware, a fork (from 'pitchfork') is a utensil, now usually made of metal, whose long handle terminates in a head that branches into several narrow and often slightly curved tines with which one can spear foods either to h ...

, the Open Group relicensed X11R6.4 under the traditional license in September 1998. The Open Group's last release came as X11R6.4 patch 3.

X.Org and XFree86

originated in 1992 from the

X386 X386 was an implementation of the X Window System for IBM PC compatible computers. It ran on systems with Intel 386 or later processors, running Unix System V-based operating systems, and supported a variety of VGA-compatible graphics cards. X38 ...

server for

IBM PC compatible An IBM PC compatible is any personal computer that is hardware- and software-compatible with the IBM Personal Computer (IBM PC) and its subsequent models. Like the original IBM PC, an IBM PC–compatible computer uses an x86-based central p ...

s included with X11R5 in 1991, written by Thomas Roell and Mark W. Snitily and donated to the MIT X Consortium by Snitily Graphics Consulting Services (SGCS). XFree86 evolved over time from just one port of X to the leading and most popular implementation and the ''de facto'' standard of X's development. In May 1999, The Open Group formed X.Org. X.Org supervised the release of versions X11R6.5.1 onward. X development at this time had become moribund; most technical innovation since the X Consortium had dissolved had taken place in the XFree86 project. In 1999, the XFree86 team joined X.Org as an honorary (non-paying) member, encouraged by various hardware companies interested in using XFree86 with Linux and in its status as the most popular version of X. By 2003, while the popularity of Linux (and hence the installed base of X) surged, X.Org remained inactive, and active development took place largely within XFree86. However, considerable dissent developed within XFree86. The XFree86 project suffered from a perception of a far too

cathedral A cathedral is a church (building), church that contains the of a bishop, thus serving as the central church of a diocese, Annual conferences within Methodism, conference, or episcopate. Churches with the function of "cathedral" are usually s ...

-like development model; developers could not get CVS commit access and vendors had to maintain extensive patch sets. In March 2003, the XFree86 organization expelled Keith Packard, who had joined XFree86 after the end of the original MIT X Consortium, with considerable ill feeling. X.Org and XFree86 began discussing a reorganisation suited to properly nurturing the development of X. Jim Gettys had been pushing strongly for an open development model since at least 2000. Gettys, Packard and several others began discussing in detail the requirements for the effective governance of X with open development. Finally, in an echo of the X11R6.4 licensing dispute, XFree86 released version 4.4 in February 2004 under a more restrictive license which many projects relying on X found unacceptable. The added clause to the license was based on the original

BSD license BSD licenses are a family of permissive free software licenses, imposing minimal restrictions on the use and distribution of covered software. This is in contrast to copyleft licenses, which have share-alike requirements. The original BSD lic ...

's advertising clause, which was viewed by the

Free Software Foundation The Free Software Foundation (FSF) is a 501(c)(3) non-profit organization founded by Richard Stallman on October 4, 1985. The organisation supports the free software movement, with the organization's preference for software being distributed ...

and

Debian Debian () is a free and open-source software, free and open source Linux distribution, developed by the Debian Project, which was established by Ian Murdock in August 1993. Debian is one of the oldest operating systems based on the Linux kerne ...

as incompatible with the

GNU General Public License The GNU General Public Licenses (GNU GPL or simply GPL) are a series of widely used free software licenses, or ''copyleft'' licenses, that guarantee end users the freedom to run, study, share, or modify the software. The GPL was the first ...

. Other groups saw it as against the spirit of the original X. Theo de Raadt of

OpenBSD OpenBSD is a security-focused operating system, security-focused, free software, Unix-like operating system based on the Berkeley Software Distribution (BSD). Theo de Raadt created OpenBSD in 1995 by fork (software development), forking NetBSD ...

, for instance, threatened to

XFree86 citing license concerns. The license issue, combined with the difficulties in getting changes in, left many feeling the time was ripe for a fork.

The X.Org Foundation

In early 2004, various people from X.Org and freedesktop.org formed the X.Org Foundation, and the Open Group gave it control of the x.org

domain name In the Internet, a domain name is a string that identifies a realm of administrative autonomy, authority, or control. Domain names are often used to identify services provided through the Internet, such as websites, email services, and more. ...

. This marked a radical change in the governance of X. Whereas the stewards of X since 1988 (including the prior X.Org) had been vendor organizations, the Foundation was led by software developers and used community development based on the

bazaar A bazaar or souk is a marketplace consisting of multiple small Market stall, stalls or shops, especially in the Middle East, the Balkans, Central Asia, North Africa and South Asia. They are traditionally located in vaulted or covered streets th ...

model, which relies on outside involvement. Membership was opened to individuals, with corporate membership being in the form of sponsorship. Several major corporations such as

currently support the X.Org Foundation. The Foundation takes an oversight role over X development: technical decisions are made on their merits by achieving rough consensus among community members. Technical decisions are not made by the board of directors; in this sense, it is strongly modelled on the technically non-interventionist GNOME Foundation. The Foundation employs no developers. The Foundation released X11R6.7, the

, in April 2004, based on XFree86 4.4RC2 with X11R6.6 changes merged. Gettys and Packard had taken the last version of XFree86 under the old license and, by making a point of an open development model and retaining GPL compatibility, brought many of the old XFree86 developers on board. While X11 had received extensions such as OpenGL support during the 1990s, its architecture had remained fundamentally unchanged during the decade. In the early part of the 2000s, however, it was overhauled to resolve a number of problems that had surfaced over the years, including a "flawed"

font In metal typesetting, a font is a particular size, weight and style of a ''typeface'', defined as the set of fonts that share an overall design. For instance, the typeface Bauer Bodoni (shown in the figure) includes fonts " Roman" (or "regul ...

architecture, a 2D graphics system "which had always been intended to be augmented and/or replaced", and latency issues. X11R6.8 came out in September 2004. It added significant new features, including preliminary support for translucent windows and other sophisticated visual effects, screen magnifiers and thumbnailers, and facilities to integrate with 3D immersive display systems such as Sun's

Project Looking Glass Project Looking Glass is a now inactive free software project under the GNU General Public License, GPL to create an innovative Graphical user interface#Three-dimensional user interfaces, 3D desktop environment for Linux, Solaris (operating sy ...

and the

Croquet project The Croquet Project is a software project that was intended to promote the continued development of the Croquet open-source software, open-source software development kit to create and deliver collaborative multi-user online application software ...

. External applications called ''

compositing window manager A compositing manager, or compositor, is software that provides applications with an off-screen data buffer, buffer for each window, then Compositing, composites these window buffers into an image representing the screen and writes the result into ...

s'' provide policy for the visual appearance. On 21 December 2005, X.Org released X11R6.9, the monolithic source tree for legacy users, and X11R7.0, the same source code separated into independent modules, each maintainable in separate projects. The Foundation released X11R7.1 on 22 May 2006, about four months after 7.0, with considerable feature improvements. XFree86 development continued for a few more years, with 4.8.0 being released on 15 December 2008.

Nomenclature

The proper names for the system are listed in the manual page as X; X Window System; X Version 11; X Window System, Version 11; or X11. The term "X-Windows" (in the manner of the subsequently released "Microsoft Windows") is not officially endorsedwith X Consortium release manager Matt Landau stating in 1993, "There is no such thing as 'X Windows' or 'X Window', despite the repeated misuse of the forms by the trade rags"though it has been in common informal use since early in the history of X and has been used deliberately for provocative effect, for example in the '' Unix-Haters Handbook''.

Key terms

The X Window System has nuanced usage of a number of terms when compared to common usage, particularly "display" and "screen", a subset of which is given here for convenience: ; device: A graphics device such as a computer graphics card or a computer motherboard's integrated graphics chipset. ; monitor: A physical device such as a

CRT CRT or Crt most commonly refers to: * Cathode-ray tube, a display * Critical race theory, an academic framework of analysis CRT may also refer to: Law * Charitable remainder trust, United States * Civil Resolution Tribunal, Canada * Columbia ...

or a flat screen computer display. ; screen: An area into which graphics may be rendered, either through software alone into system memory as with VNC, or within a graphics device, some of which can render into more than one screen simultaneously, either viewable simultaneously or interchangeably. Interchangeable screens are often set up to be notionally left and right from one another, flipping from one to the next as the mouse pointer reaches the edge of the monitor. ; virtual screen: Two different meanings are associated with this term: :* A technique allowing panning a monitor around a screen running at a larger resolution than the monitor is currently displaying. :* An effect simulated by a window manager by maintaining window position information in a larger coordinate system than the screen and allowing panning by simply moving the windows in response to the user. ; display: A collection of screens, often involving multiple monitors, generally configured to allow the mouse to move the pointer to any position within them.

Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...

-based workstations are usually capable of having multiple displays, among which the user can switch with a special keyboard combination such as control-alt-''function-key'', simultaneously flipping all the monitors from showing the screens of one display to the screens in another. The term "display" should not be confused with the more specialized jargon " Zaphod display". The latter is a rare configuration allowing multiple users of a single computer to each have an independent set of display, mouse, and keyboard, as though they were using separate computers, but at a lower per-seat cost.

Release history

On the prospect of future versions, the X.org website states:

Notes

References

* * * * * Robert W. Scheifler and James Gettys: ''X Window System: Core and extension protocols: X version 11, releases 6 and 6.1'', Digital Press 1996, * * * * * * * * * * *

External links

* {{Authority control Massachusetts Institute of Technology software Open Group standards Remote desktop Software using the MIT license Unix windowing system-related software