A window's position is defined as the coordinates of its upper left corner. These coordinates, sometimes called window coordinates, are always relative to the upper left corner of the screen or, for a child window, the upper left corner of the parent window's client area. For example, a top-level window having the coordinates (10,10) is placed 10 pixels to the right of the upper left corner of the screen and 10 pixels down from it. A child window having the coordinates (10,10) is placed 10 pixels to the right of the upper left corner of its parent window's client area and 10 pixels down from it.
The WindowFromPoint function retrieves a handle to the window occupying a particular point on the screen. Similarly, the ChildWindowFromPoint and ChildWindowFromPointEx functions retrieve a handle to the child window occupying a particular point in the parent window's client area. Although ChildWindowFromPointEx can ignore invisible, disabled, and transparent child windows, ChildWindowFromPoint cannot.