No. 10 Invoice Envelopes offer double windows for return address and mailing address. They are compatible with most standard 8-1/2" x 11" computer-generated invoices and statements. Poly windows and inside tint deliver privacy of contents. Self-sealing design makes them quick and easy to close. 24 lb. envelopes require no moisture to seal. Unique double-flap design keeps the gum from sticking before use. Simply raise the lower flap and press.