Heavyweight clasp envelopes are an excellent solution for accessible storage or filing. Whatever your home or office needs are, clasp envelopes can make staying organized easier! Great for projects, filing and mailing, the double prong clasp and reinforced eyelets are durable for repeated use. Envelopes are made of 28-lb stock that is great for extensive handling, as well as mailing. Contents are protected during use to give you assurance that it arrives unharmed. 28 lb Bond Weight Kraft Stock. Seam Type: Center; Flap Type: Square; Trade Size: #15; Security Tinted: No.