Editing X Window System (section)

==Software architecture==
{{Further|X Window System protocols and architecture|X Window System core protocol}}

[[File:X client server example.svg|thumb|Simple example: the X server receives input from a local keyboard and mouse and displays to a screen. A web browser and a terminal emulator run on the user's workstation and a terminal emulator runs on a remote computer but is controlled and monitored from the user's machine.]]

X uses a client–server model: an X server communicates with various ''client'' programs.<ref>{{Cite book |last=Quercia |first=Valerie |title=X Window System user's guide |last2=O'Reilly |first2=Tim |date=1990 |publisher=O'Reilly & Associates |isbn=978-0-937175-14-9 |series=The Definitive guides to the X Window System |location=Sebastopol, CA}}</ref> 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 [[personal computer|PC]]
* a dedicated piece of hardware

This client–server terminology{{snd}}the user's terminal being the server and the applications being the clients{{snd}}often 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]] 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 [[List of display servers#X11|Xnest]] and [[Xephyr]] support such X nesting.<ref>{{Cite web |title=xnest(1) - Linux man page |url=https://linux.die.net/man/1/xnest |website=die.net Linux Docs}}</ref>

=== 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 {{Code|ssh -X}} command to connect to the remote machine
* request a local display/input service (e.g., <kbd>export DISPLAY=''[user's machine]'':0</kbd> 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