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.
