What can be said about alternate interior angles?

Alternate interior angles are defined as pairs of angles that are formed when a transversal crosses two parallel lines. These angles are situated between the two lines but on opposite sides of the transversal. When two lines are parallel, the alternate interior angles created are equal in measure.

This property is a key part of the parallel line theorem, which states that if two parallel lines are cut by a transversal, then each pair of alternate interior angles is congruent. This congruence is crucial in various geometry problems, especially when solving for unknown angles or proving triangles congruent.

The other choices do not accurately describe the properties of alternate interior angles. For example, they are not always supplementary since they can be equal and are not confined to being acute; they can also be obtuse depending on the angle measurements involved. Therefore, the characteristic that alternate interior angles are congruent is the defining and correct statement.