The parts of an envelope that are folded, overlapped, and sealed to enclose the contents are called flaps. Flaps are usually rectangular or triangular with rounded, tapered, or pointed corners. The typical envelope consists of two side flaps, a bottom flap, and a top flap. The side flaps are folded in first with the bottom flap folded up. They are sealed where they overlap. The top flap is folded over the side and bottom flaps and sealed after inserting the envelope contents.
Envelope styles are typically defined by their style of top flaps:
- Commercial flaps are rectangular with tapered sides, rounded corners, and a shallow, rounded point.
- Wallet flaps are rectangular with slightly tapered sides and rounded corners.
- Square flaps are square or rectangular with fairly straight sides and non-rounded corners.
- Pointed flaps are triangular with a pointed or slightly rounded off corner.
There are variations to these basic flap styles, such as extra large square or wallet flaps and some types of business reply or remittance envelopes that include a tear-off section on the flap. The choice of envelope flaps not only affects appearance, but is an important consideration when using automated equipment which may require certain sizes and styles of flaps. Some automatic-insertion machines may only support nonstandard flap sizes and shapes.
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