Alternate Interior Angles

When two lines are crossed by another line (called the Transversal):

The pairs of angles on opposite sides of the transversal but inside the two lines are called Alternate Interior Angles.

In this example, these are Alternate Interior Angles:

(To help you remember: the angle pairs are on "Alternate" sides of the Transversal, and they are on the "Interior" of the two crossed lines)

Parallel Lines

If the two lines being crossed are Parallel Lines, then the Alternate Interior Angles are equal.

(Click on "Alternate Interior Angles" to have them highlighted for you.)