Envelopes, in a technical sense, are the covers used to protect letters sent by post or otherwise, to secure them from observation. Till comparatively lately the envelope was unknown, a letter being written upon a sheet, whose outside was left blank and which was folded over so as to form a space for the address. The post office sells envelopes with embossed stamps upon them. Among the early designs was the Mulready envelope, which is now an object of curiosity for collectors. New varieties of envelope are frequent.