The converse of Pascal's theorem, which states that if the three pairs of opposite sides of (an irregular) hexagon
meet at three collinear points, then the six vertices
lie on a conic, which may degenerate into a pair of lines (Coxeter and Greitzer 1967,
p. 76).
Coxeter, H. S. M. Projective Geometry, 2nd ed. New York: Springer-Verlag, p. 85, 1987.Coxeter,
H. S. M. and Greitzer, S. L. Geometry
Revisited. Washington, DC: Math. Assoc. Amer., p. 76, 1967.
