Pappus theorem
Pappus, a member of the school at Alexandria, was the last of the great creative Greek Mathematicians. This theorem, a basic theorem in projective geometry, may be considered as special case of Blaise Pascal's more general theorem (1639) on a hexagon inscribed in a conic.
More general statement of Pappus theorem is: If the vertices of a hexagon lie alternately on two lines, then the diagonal points of the hexagon are collinear.