Any paper with good opacity and foldability used for envelope production. Additionally, envelope paper accepts ink well and has good dimensional stability. Examples of papers used for envelopes are: English finish paper, manila paper, unbleached kraft paper and bond paper. Transparent windows in envelopes were initially made from glassine and cellophane and are now most often made from polystyrene.
E.J.LaBarre, Dictionary and Encyclopedia of Paper and Paper-making, Swets & Zeitlinger, Amsterdam, 1969.