Angles on opposite sides of a
transversal and inside the two lines it intersects.
Example
In these figures, angles 3 and 5 are alternate interior angles.
When lines are parallel, such as lines
d and
e, the alternate interior angles are congruent.
In the
figure on the right angles 4 and 6 have the same measure, as lines
d and
e are parallel. These angles are
congruent.
