(x,y) co-ordinate system

The co-ordinate origin of a window is its top left corner. Positive co-ordinate values indicate positions to the right and down from the origin. Negative values are also valid, and indicate co-ordinates outside the windows extent.

Each window has a position, which is the position of its origin relative to its parents origin. If its parent is a window group, then its position is effectively relative to the top left corner of the screen or display device, because window groups dont have a size.