Definition: 1.08: Plane Angle
And a plane angle is the inclination of the lines to one another, when two lines in a plane meet one another and are not lying in a straight line.
When two lines meet in a plane in a point, then we say that the lines build a plane angle.
Propositions: 2 3
Adapted from (subject to copyright, with kind permission)
- Fitzpatrick, Richard: Euclid's "Elements of Geometry"