There are four coordinate systems that are used in a windowing system:

• World CS -- The "real world" in its infinite extent that is the basis for image. Familiar units: inches, miles, light-years...

• Window CS -- This is a window into the real world with an origin and extent of your choice. This is NOT related to a window on a computer display at all! (Poor choice of terminology, but where did you think Gates and company came up with their product name?)

• Screen -- The computer display's CS. The origin is in the upper left corner with the positive X going right and the positive Y going down. Units are pixels.

• Viewport -- A region on the screen into which the above window CS is mapped. Units are pixels.