Interior and Exterior Angles

When two lines l_1 and l_2 are cut by a transversal t, eight different angles are created — four around each point of intersection. These angles are classified into two groups according to their positions with respect to lines l_1 and l_2.

• Interior angles: the angles that are between the lines l_1 and l_2.
• Exterior angles: the angles that are outside the lines l_1 and l_2.

In the diagram, all the even-numbered angles are interior angles, and all the odd-numbered angles are exterior angles.