Same-Side Angles

If two angles form a line, they are called a "linear pair," and their measures add to 180^o.  In other words, they are supplementary.

If we replace one of a pair of alternate interior angles with its linear supplement, we get a pair of same-side interior angles, and if we replace one pair of alternate exterior angles with its linear supplement, we get a pair of same-side exterior angles.

Same-side interior angles and same-side exterior angles are supplementary if and only if the two lines are parallel.

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