Double window envelopes offer a single reusable, two-way method instead of using two envelopes. Simply tear along the perforation to open, insert your response, and then seal just like a normal envelope to reuse. When you send the envelope back, it becomes a No. 9 and measures 3-7/8" x 8-7/8". 24 lb. envelopes are compatible with most standard, 8-1/2" x 11" invoices and statements. Top window (1" x 3-1/2") is positioned 1/2" from left and 2-1/2" from bottom of the envelope. The bottom window (1-1/8" x 4") is 1/2" from the left and 3/4" from the bottom. Inside tint provides privacy of contents. Adhesive is heat-resistant.