Security envelopes feature a reflex blue pattern that protects sensitive client information by obscuring the details of the contents even when held up to light. Single-window envelopes display your recipient's address for simple bulk mailings. Postage Saver Flat Mailers are made from more durable, 28 lb. paper compared to standard business envelopes. 6" x 9-1/2" are perfect for larger enclosures. Moisture-seal commercial flap and side seams provide a clean, professional look. Single window measures 1-3/8" x 4-1/2" and is positioned 7/8" from the left edge and 2-3/8" from the bottom.