An envelope is a sheet of paper given a specific shape (typically that of a rhombus or a short arm cross) which ensures that when its sides are folded about a central rectangular area a rectangular-faced enclosure is formed with an arrangement of four flaps on the reverse side with overlapping edges; although in principle the flaps can be held in place by securing the topmost flap at a single point(for example with a wax seal), generally they are pasted or gummed together at the overlaps. They are most commonly used for enclosing and sending mail (letters) through a prepaid-postage postal system.As they are made of paper they are intrinsically amenable to embellishment with additional graphics and text over and above the necessary postal indicia. This is a feature which the direct mail industry has long taken advantage of, and more recently the Mail Art movement.
An envelope is the shape of a sound which may be controlled by a synthesizer or sampler in music, derived from the above definition for electronics.
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