In any hexagon (Fig.) circumscribed around a curve of the second order (a Brianchon hexagon) the straight lines connecting the opposite corners of the hexagon intersect at one point (Brianchon's point). This theorem is dual to the Pascal theorem, and was demonstrated by Ch.J. Brianchon in 1806.
Brianchon theorem. A.B. Ivanov (originator), Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Brianchon_theorem&oldid=19270