A technique which allows a foreground scene to be superimposed on a background image from elsewhere.  This effect is obtained by recording the foreground action against a coloured (usually blue) screen or background, keying then allows the blue that has been recorded to be replaced by another image.  It is essential that nothing in the desired foreground action contains the same colour as the background.  If an actor is wearing a blue suit, for instance, most of his body will disappear when the keying replaces a blue background. Similar techniques inlude: CSO, blue screen, Ultimatte and virtual studio