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 only affected by the bits on the primary surface, not by overlays occluded by other overlays. Source color keys work on an overlay whether or not it has a z-order specified. Overlays that do not have a specified z-order behave in unspecified 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 begins with 0, just on top of the primary surface, and ends with 4 billion, just underneath the glass on the monitor. An overlay with a z-order of 2 would obscure an overlay with a z-order of 1. An overlay is not allowed to have the same z-order as another overlay.