
Protractor
A protractor is a tool used to measure angles
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.

For an animated demonstration of using the protractor see Using a Protractor
Other angle topics
General
Angle Types
Angle relationships
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