An envelope is a packaging product, usually made of flat, planar material such as paper or cardboard, and designed to contain a flat object, which in a postal-service context is usually a letter or card. 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 in the course of envelope manufacture when the sides of the sheet are folded about a delineated central rectangular area, a rectangular-faced, usually oblong, enclosure is formed with an arrangement of four flaps on the reverse side, which, by virtue of the shapes of sheet traditionally used, is inevitably symmetrical.

In 1876 William Irwin Martin published the Stationer’s Handbook. He worked for the Samuel Raynor & Company in New York. He created the first commercial sizes of envelopes and simply numbered them from 0 through 12. It was mostly for social and business stationery purposes in those days. That’s how the No. 10 envelope got its name.