There are many notions of angle used in plane geometry and just as many ways of measuring angles. The Wolfram Language can be used to compute these different angle measures.
Use VectorAngle to compute the angle between two vectors.
Use PlanarAngle to compute the angle defined by three points.
Specifying an orientation returns an angle between 0 and .
PolygonAngle can be used to measure the interior, exterior and full exterior angles of polygons.
Visualize the three angles.