Alternate Interior Angles
Since vertical angles are congruent, we can use the transitive property to replace one of two corresponding angles with its vertical angle. This gives us a pair of alternate interior angles:
The "alternate" refers to the fact that they are on alternate sides of the transversal, and the "interior" refers to the fact that they are between the two parallel lines.
As with corresponding angles, alternate interior angles are congruent if and only if the two lines are parallel.
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Date last modified: September 16, 2008.