Heath ID: I.def.10

G.18

When a straight line set up on a straight line makes the adjacent angles equal to one another, each of the equal angles is right, and the straight line standing on the other is called a perpendicular to that on which it stands.

Required for

G.19, G.20, G.21, G.23, G.24, G.25, G.26, G.27, G.28, G.30, G.31, G.32, G.34, G.35, G.36, G.38, G.39, G.42, G.43, G.44, G.45, G.46, G.47, G.48, G.49, G.50, G.51, G.52, G.53, G.54, G.55, G.56, G.57, G.58, G.59, G.60, G.61