In any keying application, you'd lay your keyed track (the foreground), in a higher track than the background track. Here, we'll put our green-screen shot, to be keyed, in video layer 2, and our computer generated background in video layer 1. This arrangement, will cause the non-transparent areas of layer 2 to be overlaid onto layer 1. To put it another way, layer 1 will only show through, where layer 2 is transparent.