If two parallel lines are cut by a transversal, then the Alternate Interior Angles are congruent.
Two pairs of alternate interior angles are formed when two parallel lines are cut by a transversal.

Here a and b are two parallel lines and c is the transversal. ∠2, ∠7 and ∠3, ∠6 are pairs of alternate interior angles. According to AIA (alternate interior angle) conjecture ∠2 and ∠7 are congruent, and ∠3 and ∠6 are congruent and can be represented as ∠2 ∠7 and ∠3 ∠6.
Solved Example on AIA Conjecture

A. ∠5 and ∠6
B. ∠1 and ∠6
C. ∠3 and ∠8
D. ∠1 and ∠4
Correct Answer: C
Step 1: Only alternate interior angles of parallel lines satisfy the AIA conjecture.
Step 2: Angles formed inside of the two parallel lines and on the opposite of the transversal are called alternate interior angles of the parallel lines.
Step 3: The alternate interior angles of the figure are ∠3 and ∠8, ∠4 and ∠5.
Step 4: So, ∠3 and ∠8 satisfies the AIA conjecture.
Q1: Which of the following pairs of angles are Alternate Interior Angles?
Q2: If two parallel lines are cut by a transversal and one Alternate Interior Angle measures 60 degrees, what is the measure of the other Alternate Interior Angle?
Q: What are Alternate Interior Angles?
A: Alternate Interior Angles are pairs of angles formed inside two lines, on opposite sides of a transversal.
Q: When are Alternate Interior Angles congruent?
A: Alternate Interior Angles are congruent if and only if the two lines cut by the transversal are parallel.