Coincidence: After two straight lines are cut by a third straight line, if two angles are on the same side of the first two straight lines and on the same side of the third straight line, they are called congruent angles.

Internal angle: After two straight lines are cut by the third straight line, if there are two angles between the first two straight lines and on both sides of the third straight line, these two angles are called internal angles.

Inner angle on the same side: after two straight lines are cut by the third straight line, if two angles are between the first two straight lines and on the same side of the third straight line, they are called inner angles on the same side.