Navigator 2.0, Internet Explorer 3.0
window.top
The top property is a read-only reference to the Window object that is the top-level window that contains window. If window is a top-level window itself, then the top property simply contains a reference to window. If window is a frame, then the top property contains a reference to the top-level window that contains the frame. Note that the top property refers to a top-level window even if window refers to a frame contained within another frame (which may itself be contained within a frame, and so on). Compare this with the Window.parent property.
Certain operations, such as setting the status and defaultStatus properties only are useful when performed on a top-level window. When JavaScript code running in a frame needs to operate on its top-level window, it can use the top property. For example, it could display a message in the message line as follows:
top.defaultStatus = 'Welcome to my Home Page!';