A nested server

Xephyr is an X server which has been implemented as an ordinary X application. It runs in a window just like other X applications, but it is an X server itself in which you can run other software. It is a very useful tool for developers who wish to test their applications without running them on their real X server. Unlike Xnest, Xephyr renders to an X image rather than relaying the X protocol, and therefore supports the newer X extensions like Render and Composite.