Pappus's Hexagon Theorem

Theorem
Let $A, B, C$ be a set of collinear points.

Let $a, b, c$ be another set of collinear points.

Let $X, Y, Z$ be the points of intersection of each of the straight lines $Ab$ and $aB$, $Ac$ and $aC$, and $Bc$ and $bC$.

Then $X, Y, Z$ are collinear points.

Proof

 * PappusHexagonTheorem.png

Also known as
This theorem is also known just as Pappus's Theorem.

Also see

 * Pascal's Theorem
 * Desargues' Theorem