a and b are parallel lines. They are cut by transversal c. Drag the blue points on c to move the transversal. [br]How does this affect the angles formed by the intersection of a and b with c? [br]Which angles are always congruent? [br]Which angles are always supplementary?[br][br]Note: Do not allow the blue points to fall inside the parallel lines or the angles will disappear!

Angles on opposite sides of a common arm are called adjacent angles.[br][i]AD[/i] is the common arm.