X2x joins a pair of X displays together, as if they were a single multiheaded display. The pointer can be warped between displays, or, depending on how you start x2x, can slide from one display to the other when it runs off the edge of the screen. Keyboard focus also moves between displays in the way you'd expect, and the X selection propagates around. This is useful when you have two displays on the same desk, as you can use a single keyboard and mouse to control both. (The displays being controlled remotely must support the XTEST extension. The servers shipped by Debian support this.)