All drawing in a window takes place in the context of the window's coordinate system. Locations of points in the window are described by POINTL structures, which contain an x and a y coordinate for the point. The lower-left corner of a window always has the coordinates (0,0).

The WinMapWindowPoints function converts the coordinates of points from one window-coordinate space to those of another window-coordinate space. If one of the specified windows is HWND_DESKTOP, the function uses screen coordinates. This function is useful for converting window coordinates to screen coordinates or the other way around.