I'm reading Wikipedia about X11 and it says that:

In its standard distribution it is a complete, albeit simple, display and interface solution which delivers a standard toolkit and protocol stack for building graphical user interfaces on most Unix-like operating systems...

But later it says that:

X primarily defines protocol and graphics primitives - it deliberately contains no specification for application user-interface design, such as button, menu, or window title-bar styles.

So, does X11 provide widgets like a button or a window panel/frame, etc or not? What is a graphic primitive? What does X11 provide exactly?

It is also stated that:

X does not mandate the user interface; individual client programs handle this. Programs may use X's graphical abilities with no user interface.

What does this mean?

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    I think you should probably go to the conversation page corresponding to that page in Wikipedia and start a conversation aiming for the authors to clarify their article. Apr 13, 2016 at 12:47
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    Wikipedia is great, but it caters "power-users" first and tend to give detailed information first instead of short and concise answers first, then detailed informations.
    – X.LINK
    Jun 29, 2021 at 12:04
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    X11 is also a OSI level 7 protocol ie the format of the datastream over the network and its semantics. An application can be programmed using libX11 or libxcb directly (a basic low level programming API for a GUI.) libX11 uses the network format to communicate with a server ie a program having command of a screen and access to a data stream from keyboard(s) and mouse(s.) libXt is a library which together with a number of widgets implements a more high-level API. The older versions of Gtk replaces libXt but depends on libX11 - it doesn't communicate directly with the server. Jun 29, 2021 at 18:39
  • @Stefan Skoglund But if an application does what you say - using libX11 or libxcb - then aren't they simply using X's internal fallback libraries to build their user interface ? The statement Programs may use X's graphical abilities with no user interface clearly implies the absurd scenario of an app without any user interface seeking to use X . . .
    – Trunk
    Jul 15, 2021 at 13:03
  • I've edited the absurd sentence in the Wikipedia page. Feel free to improve it.
    – Trunk
    Jul 15, 2021 at 13:12

4 Answers 4


Like many words, “X11” can have multiple meanings.

“X11” is, strictly speaking, a communication protocol. In the sentences “X primarily defines protocol and graphics primitives …” and “X does not mandate the user interface …”, that's what X refers to. X is a family of protocols, X11 is the 11th version and the only one that's been in use in the last 25 years or so.

The first sentence in your question refers to a software distribution which is the reference implementation of the X11 protocol. The full name of this software distribution is “the X Window System”. This distribution includes programs that act as servers in the X11 protocol, programs that act as clients in the X11 protocol, code libraries that contain code that makes use of the X11 protocol, associated documentation, resources such as fonts and keyboard layouts that can be used with the aforementioned programs and libraries, etc. Historically, this software distribution was made by MIT; today it is maintained by the X.Org Foundation.

The X11 protocol allows applications to create objects such as windows and use basic drawing primitives (e.g. fill a rectangle, display some text). Widgets like buttons, menus, etc. are made by client libraries. The X Window System includes a basic library (the Athena widget set) but most applications use fancier libraries such as GTK+, Qt, Motif, etc.

Some X11 programs don't have a graphical user interface at all, for example command line tools such as xset, xsel and xdotool, key binding programs such as xbindkeys, etc. Most X11 programs do of course have a GUI.

  • xset is just a CLI command to set mouse, screen and sound parameters for the X Window System. xsel and xdotool are not installed by default and just seem to do on the CLI what is usually done via GUI. I doubt if these are what the Wikipedia page author had in mind when they wrote the absurd sentence.To me, it's more likely to refer to the internal X Athena library and applications that build a basic GUI with this. Though not proprietary, this is still a user interface and will therefore need X. So the sentence - not unusually for IT documentation - remains absurd !
    – Trunk
    Jul 15, 2021 at 13:38
  • @Trunk Do you mean, "Programs may use X's graphical abilities with no user interface."? I think this may refer to e.g. the fact that programs can just draw directly into a window without using a toolkit library (Athena, GTK+, Qt, etc.). A classic example is Xeyes. There are also programs that draw on the root window (although for various reasons, that no longer has a visible effect with some modern desktop environments). Depending on whether you still count those as having a UI, the sentence may or may not be reasonable.
    – ddawson
    Jul 19, 2021 at 13:51

X11, aka X.org Foundation Windowing is basically used by other desktop environments like KDE and GNOME, among many others, to provide the abstract interface to managing a GUI. Without relying on X, KDE/GNOME/others would have to write the code to do low-level coding things themselves. Instead, KDE/GNOME communicate back and forth with X (it runs as a server process that 'clients' connect to).

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    Also this help explain it, as well as provide some insight into why Wayland and other X replacements are being built: art.net/~hopkins/Don/unix-haters/x-windows/disaster.html Apr 13, 2016 at 13:42
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    That is a rant. Feb 29, 2020 at 21:02
  • And it also in another way explains Ken Olsen's characterization of UNIX as snake oil peddler's products, but that IS capitalism ! The claim is design for MOTIF and run everywhere, nowadays it's more like design for MS windows and run everywhere (this a monopoly not a market.) Ken Olsen knew very well that design-for-MOTIF and run-on-everything is not in the producers interest - ergo it won't really happen. Feb 29, 2020 at 21:06
  • Unfortunately this answer starts by conflating concepts: "X11, a.k.a. X.org Foundation Windowing". Gilles' answer explains how they're different from each other, which is the core of the question.
    – Quasímodo
    Apr 30, 2021 at 22:09

Some concepts:
- X Windows System Windowsing system. (make it possible for moving windows on display device and interacting with mouse and keyboard)
- X.Org Server An open-source implementation of X Windows System by X.Org Foundation.
- xterm terminal emulator for X Windows System.
- X Client: an application runs on an X server. X windows system requires the clients and server to operate separately. Applications, such QT, GTK and other X clients need the cooperation from X server to work successfully.

Previously, I also feel very confused on this problem. Initially, I installed linux system aside with windows, for many times I encountered issues with Xorg errors.
Recently, I use xclip on remote server. The software needs an environment variable DISPLAY to be set, which is not the case in my remote server. There is no monitor, mouse or keyboard, and the variable is managed by X server.


The X11 window system's different server programs (the process controlling the device ie the screen(s) with kbd(s) and mouse(s)) can support extensions to the normal X11 on the wire format. Wire format: my wording for the different types of messages which can be transported over IP (nothing prevents transporting X11 messages over IBM's SNA for example) between a client process running on a machine and the server program.

One extension to an X11 server program is DisplayPostScript.

The usage of DisplayPostscript in NeWS (Sun/Sony) meant that the server had a complete and rather nice (Display)Postscript support built in. This support could be expoloited by a client program to run postscript programs directly inside the server.

This could be employed by for example oil exploration programs to with the same API as if the device were a high-end photosetter/printer, display results on the scientist's display (CRT)

Adobe hasn't erased DisplayPostScript (July 2021) from its systems so read :

  • How is this about X11? Jun 29, 2021 at 10:51
  • It concerns the flexibility of X11 and its usage. The DisplayPostscript would nowadays maybe instead be done using HTML5. Jun 29, 2021 at 18:43
  • Make the answer clear, without need to read comments. Jul 1, 2021 at 16:52

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