Patented release strip makes these open-end catalog envelopes easy to open. Convenient Flap-Stik closure does not require moisture to seal. First class envelopes made of DuPont Tyvek won't rip, tear or puncture. They are much stronger and more secure than traditional paper envelopes. Water-resistant and moisture-resistant envelopes are lightweight to save on postage costs. Easy to use. Simply peel and seal.