Key to Geometry offers a non-intimidating way to prepare students for formal geometry as they do step-by-step constructions. Using only a pencil, compass, and straightedge, students begin by drawing lines, bisecting angles, and reproducing segments. Later they do sophisticated constructions involving over a dozen steps and are prompted to form their own generalizations. When they finish, students have been introduced to 134 geometric terms and are ready to tackle formal proofs. Book 4 covers Perpendiculars.