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Place your mail inside the envelope. Lift the clasp arms. Wet or peel the flap if necessary. Push the metal clasps through the hole in the flap. Flatten and secure the arms. Affix postage.

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Flip the envelope open and slide your mail into the opening. Place the top edge of the document nearest the opening if possible. The most common type of clasp envelope has a butterfly clip with two metal arms. Unfold these so they stand straight up. You may need to pry them up using your fingernails. If your envelope has two circles and a string instead, just close the flap and wrap the string tightly around the other circle instead. If your envelope has a strip of paper along the edge of the flap, peel this off to reveal the adhesive. If your flap has a lick-able strip, wet this slightly to make it sticky. Lower the flap and adjust the metal arms so that they are positioned right beneath the hole. Push the hole over both arms. Seal the flap against the body of the envelope by running over the edge with your thumb. Push the clasp arms outward again and flatten them against the flap. To prevent the clasp from catching on other mail, put a piece of tape over the arms. Apply the appropriate postage in the upper right-hand corner of the envelope, and your envelope is ready to mail. The exact postage required depends on the size and weight of the envelope, as instructed by your local post office. If you can't find instructions, just take the envelope to the post office and pay for postage over-the-counter.