http://www.math.tamu.edu/~dallen/history/euclid/euclid.html Euclid's Elements Book 1 Postulate 5 http://www.math.tamu.edu/~dallen/history/euclid/euclid.html http://www.math.tamu.edu/~dallen/history/euclid/euclid.html Euclid's Elements Book 1 Postulate 5