A two-way envelope includes a front panel with a window opening and a back panel connected to the front panel along a lower fold line. A pair of side flaps extend from the front panel and are secured to an inner surface of the back panel. An initial seal flap extends from an upper edge of the front panel...http://www.google.fr/patents/US4565317?utm_source=gb-gplus-shareBrevet US4565317 - Two-way envelope with inside return seal flap
