【RelationShip】
Typically, the browser creates one window object when it opens an HTML document. However, if a document defines one or more frames—that is, contains one or more frame or iframe tags—the browser creates one window object for the original document and one additional window object for each frame.
通常，浏览器打开一个HTML文档的时候建立一个window。可是，如果一个文档定义一个或多个frame，那么，浏览器为原始文档建立一个window，并为每一个frame建议一个window。
These additional objects are child windows of the original window。
这些对象是原始窗口的子窗口。
