Overlay z-order determines the order in which overlays clip each other, enabling a hardware sprite system to be implemented under DirectDraw. Overlays are assumed to be on top of all other screen components. Destination color keys are affected only by the bits on the primary surface, not by overlays occluded by other overlays. Source color keys work on an overlay whether or not a z-order was specified for it. Overlays that do not have a specified z-order behave in unpredictable ways when overlaying the same area on the primary surface. Finally, overlays without a specified z-order are assumed to have a z-order of 0. The possible z-order of overlays ranges from 0, which is just on top of the primary surface, to 4 billion, which is as close to the viewer as possible. An overlay with a z-order of 2 would obscure an overlay with a z-order of 1. No two overlays can have the same z-order.