Protractor

A protractor is a graduated instrument for measuring or drawing planar angles, commonly a semicircular scale from $0^{\circ }$ to $180^{\circ }$. Place its centre at the vertex, align its zero baseline with one ray, and read where the other ray crosses the scale. A $60^{\circ }$ reading represents $\pi /3$ radians. A device with oppositely directed scales requires choosing the scale whose zero matches the aligned baseline.