An application can allow the system to calculate the initial size or position of a top-level window by specifying CW_USEDEFAULT in CreateWindowEx. If the application sets the window's coordinates to CW_USEDEFAULT and has created no other top-level windows, the system sets the new window's position relative to the upper left corner of the screen; otherwise, it sets the position relative to the position of the top-level window that the application created most recently. If the width and height parameters are set to CW_USEDEFAULT, the system calculates the size of the new window. If the application has created other top-level windows, the system bases the size of the new window on the size of the application's most recently created top-level window. Specifying CW_USEDEFAULT when creating a child or pop-up window causes the system to set the window's size to the default minimum window size.