No. 9 invoice envelopes offer double windows for return address and mailing address. Compatible with most standard 8-1/2" x 11" computer-generated invoices and statements. Top window (1-3/16" x 3-1/2") is positioned 3/8" from left and 2" from the bottom of the envelope. Bottom window (1" x 4") is 3/8" from the left and 1/2" from the bottom. 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.