Folding a Rectangle into an Envelope
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You can construct an envelope by folding a paper rectangle. The face of the envelope has half the area of the rectangle.
Contributed by: Izidor Hafner (January 2016)
Open content licensed under CC BY-NC-SA
 D. Wells, The Penguin Book of Curious and Interesting Puzzles, Penguin Books, New York, 1993 pp. 53, 238, and 304.
"Folding a Rectangle into an Envelope"
Wolfram Demonstrations Project
Published: January 4 2016