Euclid's Elements Book 1, Proposition 29

Project Euclid presents Euclid's Elements, Book 1, Proposition 29 "A straight line falling on parallel straight lines makes the alternate angles equal to one another, the exterior angle equal to the interior and opposite angle, and the sum of the interior angles on the same side equal to two right angles." Presented by Zachary Calhoun, St. John's College, Class of 2015