http://www.freepatentsonline.com/EP1444140.html

WINDOW ENVELOPE

Document Type and Number:
European Patent EP1444140

The invention relates to a window envelope (1) having two flat sides, which form the front and rear sides (2, 3) thereof and are joined to each other at the edges thereof. Said window envelope also comprises an address window (5), which is made of a transparent material and which is distant from the window envelope (1) edges. Said window envelope is characterized in that the material of the address window (5) is extended to one of said edges and continues at least partially on the rear side of said window envelope (1) in a coincident manner.

Yep! Sagt man. Auch für Adressfenster in Emails.. Fensterumschlag =window envelope