High Quality Content by WIKIPEDIA articles! Xnest is an X Window System server that shows its output in a window. In other words, Xnest opens a window that works like another screen in which the user can open windows, etc. At the protocol level, Xnest works like a client of the server hosting the window, and as a server with respect to the applications that opens windows within the Xnest session. Xnest can be used to run a virtual desktop of another computer within a window. Xnest is also used for server debugging and for testing that applications work correctly at different screen sizes. Indeed, the user can choose the size of the Xnest window, which is also the size of the virtual screen it contains. This way, one can for example start an Xnest window with the size of the screen of a PDA to test whether an application works correctly on the screen of that device.