Security business envelopes with a window are made with a material that inhibits the growth of mold & mildew to protect the product. Redi-Strip on the flap provides a clean, quick seal immediately upon contact. Simply peel off the adhesive strip & press to seal. The peel-off strip protects the adhesive from dust for a longer shelf life. No. 10 envelopes are tinted to protect the privacy of your contents.