XRandR

RandR ("resize and rotate") is a written as an extension to the protocol. XRandR provides the ability to resize, rotate and reflect the root window of a screen. RandR is also responsible for setting the screen refresh rate.

An implementation of RandR is part of the X.Org Server.

A user can typically use applications with a graphical front-end provided by the desktop environment to control RandR, but the additional command line tool xrandr exists. xrandr tells the display controller what resolution and refresh rate it should output on which of its outputs (e.g. VGA1, HDMI3). The name of the output is determined by the device driver for the display controller (KMS driver).

The initial X11 design did not anticipate the need for dynamic resizing and it was necessary to restart the X display server to bring about the changes. However, XFree86 has, since its first release, allowed the user to change the screen resolution on the fly without changing the desktop size. The RandR extension framework brought the ability to change display characteristics without restarting the X session. The extension framework allows laptops and handheld computers to change their screen size to drive external monitors at different resolutions than their built in screens.

The RandR extension was initially implemented in the XFree86's TinyX server, and partial support for version 1.0 was added to XFree86 4.3.

RandR 1.2 permits only one virtual screen per display device. It is not possible to assign each monitor on a device to a different screen (sometimes called "Zaphod" mode), or to combine monitors from multiple devices into a single screen. One practical limiting effect of this is that it is not possible to run a different WM on each monitor, since window managers are limited to one per screen.