Pascal's mystic hexagon

If a hexagon ADBFCE (not necessarily convex) is inscribed in a conic section (in particular a circle), then the points of intersections of opposite sides (AD with FC, DB with CE and BF with EA) are collinear. This line is called the Pascal line of the hexagon. A special case is when the conic degenerates into two lines; the theorem still holds but is then usually called Pappus' theorem.