When two lines are cut by a transversal, the pair of angles formed interior of the two lines and on the opposite of the transversal is called Alternate Interior Angles.

More About Alternate Interior Angles:

If two parallel lines are cut by a transversal, the alternate interior angles are congruent

Examples of Alternate Interior Angles

In the figure shown, l is the transversal that cut the pair of lines. Angles a and d and angles b and c are alternate interior angles.

Video Examples: Alternate Interior Angles

Solved Example on Alternate Interior Angles

Ques: Find the measure of angle m in the figure shown.

Choices:

A. 120^{o}
B. 180^{o}
C. 60^{o}
D. Insufficient information

Correct Answer: C

Solution:

Step 1: 120^{o} + n = 180^{o} [Straight angle.]
Step 2: n = 60^{o} [Solve for n.]
Step 3: n = 60^{o} implies m = 60^{o}, because m and n are alternate interior angles and so they are congruent.