If the three straight Lines joining the corresponding Vertices of two Triangles and all meet in a point (the Perspective Center), then the three intersections of pairs of corresponding sides lie on a straight Line (the Perspective Axis). Equivalently, if two Triangles are Perspective from a Point, they are Perspective from a Line.

Desargues' theorem is dual to Pappus's Hexagon 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. 70-72, 1967.

Ogilvy, C. S. *Excursions in Geometry.* New York: Dover, pp. 89-92, 1990.

© 1996-9

1999-05-24