GREG uses a graphic library with a virtual memory storage for all normal plot actions. Accordingly, the ``plot'' is a notion entirely independent of any graphic device, and it can be generated without any. The only thing necessary to define a plot is the Plot Page size in a conventional unit called the Physical Unit. Plot coordinates on the Plot Page are called Physical Coordinates, or World Coordinates. By convention, the Physical Unit will be called a centimeter ; this is meaningful because when you produce a drawing from a metacode file using a specific driver such as PLXY, a Physical Unit will match a centimeter if you use the /EXACT option.