Lindgren used a quadrilateral slide to transform one triangle (as a trapezoid with a top edge of length 0) to a trapezoid with the same angles. A quadrilateral slide (or Q-slide) transforms one quadrilateral to another with the same angles. The notion was introduced by Lindgren (1956).
Harry Lindgren (1912-1992) was born in England and emigrated to Australia in 1935. He was for many years the world's leading expert in geometric dissections (Frederickson 1997, 183).
G. N. Frederickson, Dissections: Plane & Fancy, New York: Cambridge University Press, 2002 pp. 41–42.
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