If , , and are three points on one Line, , , and are three points on another Line, and meets at , meets at , and meets at , then the three points , , and are Collinear. Pappus's hexagon theorem dual to Desargues' Theorem according to the Duality Principle of Projective Geometry.

**References**

Coxeter, H. S. M. and Greitzer, S. L. *Geometry Revisited.* Washington, DC: Math. Assoc. Amer., pp. 73-74, 1967.

Ogilvy, C. S. *Excursions in Geometry.* New York: Dover, pp. 92-94, 1990.

Pappas, T. ``Pappus' Theorem & the Nine Coin Puzzle.'' *The Joy of Mathematics.* San Carlos, CA: Wide World Publ./Tetra,
p. 163, 1989.

© 1996-9

1999-05-26