[derefr]

My kitchen has a switch on a little stub of a wall, the other side of which is part of the living room. The living-room-side switch controls the kitchen lights, and vice-versa. It makes sense when you realize that you can use them to do a sort of "airlock" process, turning on the light in the room ahead of you, entering it, then turning around and switching off the lights in the room behind you.