Lindgren's Dissection of a Quadrilateral to a Triangle
This Demonstration shows Lindgren's dissection of a quadrilateral to a triangle. The dissection starts with "quadrilateral" selected and the "move" slider to the right. To see the progress of the dissection, move the slider to the left, select "triangle," and move the slider back to the right.
Lindgren constructed this dissection using the intersection of a quadrilateral and a triangular strip.
 G. N. Frederickson, Dissections: Plane & Fancy, New York: Cambridge University Press, 2002 pp. 139–140. www.cs.purdue.edu/homes/gnf/book.html.