A child window is created by applying the window style Style::Child. The parent window handle must be supplied to the constructor via the parameter parent.
Child windows position themselves relative to the parent. Thus, when setting the position of a child, the specified position and size are relative to the parent's client area.
Not only does a child window position itself relative to its parent, a child window is confined to lie within the client area of the parent. Any portion of a child that lies outside the client area of its parent is clipped away when the window is drawn.