Angles with Parallel Lines
When a transversal cuts across parallel lines, the eight angles created can be divided into interior angles (angles 3, 4, 5, 6) which fall between the parallel lines, and exterior angles (angles 1, 2, 7, 8) which fall outside the parallel lines.
Corresponding angles are angles in the same position relative to both parallel lines (like angles 1 & 5, 2 & 6, 3 & 7, and 4 & 8). These angles are always congruent when the lines are parallel.
Alternate interior angles are angles that lie within the parallel lines but on opposite sides of the transversal (angles 3 & 6, 4 & 5). These are also congruent. Consecutive interior angles are angles inside the parallel lines on the same side of the transversal (like angles 4 & 6, 3 & 5). These angles are supplementary - they add up to 180°.
Remember This: When parallel lines are cut by a transversal, corresponding angles and alternate interior angles are equal, while consecutive interior angles add up to 180°.