A protractor is a transparent plastic tool designed to measure angles. They are usually semicircular but also are available in a full circle 360° version.
| Origin | The small hole near the bottom of the protractor is aligned over the vertex of the angle to be measured. |
| Baseline | The line along the bottom of the protractor is aligned along one of the angle's legs.
Note: It is not the edge of the protractor body itself, but parallel to it. |
| Scales | The angle of the other leg is read from one of the two scales, whichever is more convenient. |