Pappus's Theorem

A classical projective theorem: given two distinct lines and three points A,B,C on the first and A',B',C' on the second, the three intersection points of cross‑connections (for example, AB'∩A'B, AC'∩A'C, BC'∩B'C) are collinear. The theorem holds in projective planes coordinatizable over a commutative field (Pappian planes).