- Seal Adhesive
The strip of adhesive along the edge of the open top flap used to seal the envelope. Some adhesive requires moistening (such as with water or by licking the edge) and other types require two gum surfaces that press together and seal without moisture. A peel and stick adhesive has a strip of waxy paper covering the adhesive until the envelope is ready for sealing. - Back Gum
This a stronger adhesive used to seal the side and bottom flaps of the envelope during manufacturing. - Clasp
Some booklet and catalog envelopes have metal or plastic fasteners with or without re-moistenable gum that allows the envelope to be opened and closed repeatedly. - String-and-Button
For repeated opening and closing some envelopes have a metal or paper button with a string that wraps around the button to hold the top flap closed. Expanding envelopes may have elastic strings that stretch around the entire envelope to accommodate the varying size of the contents and hold the envelope closed. - Tamper-Evident Seal
Envelopes bearing confidential or sensitive contents can have a special seal that indicates if it has been broken. A window on the flap reveals the word "Opened" if the flap has been opened and reclosed. Or, the top flap may contain a perforated strip that can only be peeled off once (non re-sealable) to reveal the envelope contents.
Paper Weight
Standard envelopes styles and sizes use specific paper weights although a designer can request custom paper choices. US Air Mail envelopes use a lighter 13-16 lb. paper to help keep the cost of overseas mailings lower. Some types of clasp or storage envelopes that are handled a lot, such as in offices, may use heavier 32 lb. to 40 lb. paper. 20 lb. to 28 lb. is typical for most Commercial, Baronial, and A-Style envelopes.
