Abstract: An improved envelope suitable for interleaving with the pages of a magazine and to be sewed or otherwise bound to the spine of the magazine. The envelope has a front and a back and an entrance for receiving sheet or other material. The back has a lip projecting outwardly from the entrance and a relatively large flap hingedly connected to the outer margin of the lip. When the envelope is moved through a conventional envelope stuffing machine, the lip is held clamped against upward movement as the flap is beneath and contiguous to the back, thereby permitting a suction or other force exerted on the front to lift the latter and thereby open the entrance to facilitate insertion of the material into the envelope.