The 415 tape from 3M is a high quality double sided transparent tape most often used for creating Mylar envelopes, as well as mounting photographs and labels, and other applications where a narrow width high tack double sided tape is required. Rolls are all 36 yards.
Mylar envelopes can be created with two sheets of polyester and a narrow width 415 tape appropriate for the size envelope being made. The tape is run along three edges of one sheet of Mylar, the backing from the tape is removed, and the second sheet of Mylar is adhered down on top. This will create an envelope sealed on three sides.