C.I.A. - This is not the Central Intelligence Agency! This acronym will help you to understand angles on parallel lines.[br][br]C - corresponding angles[br]I - Interior angles (or supplementary angles)[br]A - Alternate angles[br][br]Take a look at the graphic below.[br]Corresonding angles are marked in green.[br]Interior angles are marked in red.[br]Alternate angles are marked in blue.[br][br]Corresponding angles are always equal (on a parallel line).[br]Alternate angles are always equal (on a parallel line)[br][br]Interior angles (on a parallel line) always add up to 180[math]^\circ[/math]