Navigator 2.0, Internet Explorer 3.0
window.parent
The parent property is a read-only reference to the Window object that contains window. If window specifies a top-level window, then parent refers to the window itself. If window is a frame, then the parent property refers to the top-level window or frame that contains window.
The parent property is generally only useful for frames rather than for top-level windows. With a function like the following, you can use the property to determine whether a given Window object is a top-level window or a frame:
function is_toplevel(w) { return (w.parent == w); }