An envelope is a packaging product, usually made of flat, planar material such as paper or cardboard, designed to contain a flat object such as a letter. The traditional type is made from a sheet of paper cut to one of three shapes : the rhombus (also referred to as a lozenge or diamond), the short-arm cross, and the kite. These designs ensure that when the sides of the sheet are folded about a delineated central rectangular area, a rectangular-faced, usually oblong, enclosure is formed with a symmetrical arrangement of four flaps on the reverse side with overlapping edges.
When the folding sequence is such that the last flap to be closed is on a short side it is referred to in the commercial envelope manufacturing industry as a '"pocket"'. 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.
An aerogram is related to a lettersheet, both being designed to have writing on the inside to minimize the weight. Any handmade envelope is effectively a lettersheet because prior to the folding stage it offers the opportunity for writing a message on that area of the sheet which after folding becomes the inside of the face of the envelope.
The "envelope" used to launch the Penny Post component of the British postal reforms of 1840 was a lozenge-shaped lettersheet. But if desired a separate letter could be enclosed and postage remained one penny provided the combined weight did not exceed half an ounce (about 13 grams).
A "return envelope" is a preaddressed, smaller envelope included as the contents of a larger envelope. Some envelopes are designed to be reused as the return envelope, saving the expense of including a return envelope in the contents of the original envelope. The direct mail industry makes extensive use of return envelopes as a response mechanism.
Up until 1840 all envelopes were made by hand, by individually cutting the appropriate shape out of an individual rectangular sheet. In that year George Wilson in the U.K. patented the method of tessellating (tiling) a number of envelope patterns across and down a large sheet, thereby reducing the overall amount of waste produced per envelope when they were cut out. In 1845 Edwin Hill and Warren De La Rue obtained a patent for a steam-driven machine which not only cut out the envelope shapes but creased and folded them as well. (Mechanised gumming had yet to be devised.)
As envelopes 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.
Most of the over 400 billion envelopes of all sizes made worldwide are machine-made. The envelope-machine making industry is dominated internationally by Winkler and Dunnebier.