"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 ...
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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 the envelope structure to be made 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." - (en.wikipedia.org 30.03.2022)
