# Pappus axiom

If $l$ and $l'$ are two distinct straight lines and $A,B,C$ and $A',B',C'$ are distinct points on $l$ and $l'$, respectively, and if none of these is the point of intersection of $l$ and $l'$, then the points of intersection of $AB'$ and $A'B$, $BC'$ and $B'C$, $AC'$ and $A'C$ are collinear.

Figure: p071140a

The truth of Pappus' axiom is equivalent to the commutativity of the skew-field of the corresponding projective geometry. The Desargues assumption is a consequence of Pappus' axiom (Hessenberg's theorem), and at the same time Pappus' axiom is a degenerate case of the Pascal theorem. The axiom was proposed by Pappus (3rd century).