2. In order to distinguish these angles, we call the smaller one of the two pairs of vertex angles the angle between L 1 and L2.
3. The included angle is greater than or equal to zero and less than or equal to 90 degrees.
4. Angle is widely used in geometry and trigonometry.
5. Euclid, the father of geometry, once defined an angle as the relative inclination of two non-parallel straight lines in a plane. Proclos thinks that angle may be a trait, a quantifiable quantity, or a relationship. Oldham thinks that an angle is a deviation from a straight line, and Cabus of Antioch thinks that an angle is a space between two intersecting straight lines.
6. Euclid thinks that an angle is a relationship, but his definitions of right angle, acute angle or obtuse angle are quantitative.