Euclid's Elements Book 1, Proposition 14

Project Euclid presents Euclid's Elements, Book 1, Proposition 14 "If with any straight line, and at a point on it, two straight lines not lying on the same side make the sum of the adjacent angles equal to two right angles, then the two straight lines are in a straight line with one another." Presented by Scout Burchill, St. John's College, Class of 2015