A window's size and position are expressed as a bounding rectangle, given in coordinates relative to the screen or the parent window. The coordinates of a top-level window are relative to the upper left corner of the screen; the coordinates of a child window are relative to the upper left corner of the parent window. An application specifies a window's initial size and position when it creates the window, but it can change the window's size and position at any time. For more information about bounding rectangles, see Filled Shapes.