Constructions
58. The 60° Blueprint · Building perfect angles with compass & ruler
The constructed angle ∠COA always remains exactly 60°.
A 60° angle can be constructed exactly with compass and straightedge: from vertex O, draw an arc of radius r that crosses the base ray at B. With the same radius, an arc centred at B crosses the first arc at C. Then ∠COA is exactly 60°, because OB = BC = OC makes triangle OBC equilateral.