An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter or card.

Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a short-arm cross or a kite. These shapes allow for the creation of the envelope structure by folding the sheet sides around a central rectangular area. In this manner, a rectangle-faced enclosure is formed with an arrangement of four flaps on the reverse side

Envelope is designed for computer generated invitations, greeting cards, announcements and more. Sized to hold a 8-1/2″ x 11″ sheet folded in quarters.

They  are made of paper, they are intrinsically amenable to embellishment with additional graphics and text over and above the necessary postal markings.

This is a feature that the direct mail industry has long taken advantage of—and more recently the Mail Art movement. Custom printed envelopes has also become an increasingly popular marketing method for small business.