Quality Park® Security Envelopes have a reflex blue pattern that protects sensitive client information because it obscures the details of the contents even when held up to light. Single window envelopes display the recipient s address for simple bulk mailings. These postage saver flat mailers are made from a more durable 28-lb paper than standard business envelopes so they will stand up to mailings. Seam Type: Diagonal; Flap Type: Commercial; Trade Size: #6 5/8; Security Tinted: No.