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. This proposition defines further the characteristics of a straight line, and it allows one to judge the consistencies of a straight line. If one were to construct a straight line and split the line in two with another line the angles will be equal to two right angles, if it is indeed a straight line.

Video instructed by myself. Note: I am greatly disappointing by this performance, and hope to improve it soon, for now let this serve as a temporary instruction of proposition 14, book 1.
