A rectangular envelope formed from a single sheet of paper has a front panel with a window and foldable flaps on each edge. A first enclosure in the envelope has an address visible through the window and three of the flaps hold the enclosure in place and are loosely held in place while the fourth flap...http://www.google.fr/patents/US4089418?utm_source=gb-gplus-shareBrevet US4089418 - Returnable mail envelope
