Alternate interior angles are two pairs that lie in between the 2 parallel lines (thus, "interior") but lie on opposite sides of the transversal (the line intersecting the 2 parallel lines)(thus, "alternate").
Alternate interior angles are always congruent.
There is a pair that are acute and congruent, and a pair that are obtuse and congruent.
The only time that does not occur is when the transversal is perpendicular to the parallel lines, which would create right angles everywhere. That is also the only instance in which alternate interior angles will also be supplementary (sum to 180 degrees).