Abstract: The present invention is related to an envelope (1) provided with at least one window (2), said envelope being intended for double-use as a reply envelope, too. Said window is provided with a detachable flap (3), whereby said window of the envelope can be made larger by tearing off said flap from the envelope. Subsequently, the envelope can be reused for mailing a reply whereas the enlarged window of the envelope will now display the receiver name and address information preprinted on the reply letter, plus the prepaid reply postage markings and other possibly desirable information.