Quality Park catalog mailing envelopes are perfect for shipping, storing or organizing. Heavyweight 28-lb paper withstands extensive handling at home, in the office or on the mail route. These envelopes are perfect for small flat or larger folded documents. A wide, heavily gummed commercial flap seals tight with moisture to keep inserts secure. Center seams give added strength for heavy contents. 28 lb Bond Weight Kraft Stock. Seam Type: Center; Flap Type: Square; Trade Size: #13 1/2; Security Tinted: No.