This listing is for cardboard mailers specially sized to fit cassette tapes! They are made of 200# test cardboard, and fit one standard sized cassette. They feature a lock in tab to hold box closed, and have edges that hang over to provide extra protection! The boxes measure 5-1/2" x 3-1/4" x 1-1/8" (139.7mm x 82.55mm x 28.6mm).