Business Source - BSN42101 Heavy-duty, all-purpose mailing envelopes are ideal for mailing documents, catalogs, direct mail, promotional material, brochures and more. 28 lb. opaque Kraft construction delivers privacy of contents. Open-ended style allows you to easily insert and remove items. Moisture-activated, gummed flap provides a secure seal. 10" x 13" size is perfect for documents without folding.