A *plane angle* is the inclination to one another of two lines in a plane which meet one another and do not lie in a straight line.

The two lines are meant to emanate from the same point; two intersecting lines will actually make four angles.

The concept is also a difficult one, and, surprisingly, broader than our modern concept of angle.

As can be seen from the next definition of rectilinear angle, angles do not have to have straight sides; they can have curves as sides. The size of the angle does not depend on the length of the sides, but is determined only by how the two sides meet. In the |