1. The midpoint of a segment is the point that divides the segment into two congruent segments if and only if a point divides the segment into two congruent segments, then it is the midpoint of the segment.
2. Two angles are vertical angles when their sides form two pairs of opposite rays if and only if two angles form vertical angles when their sides form two pairs of opposite rays.
3. Adjacent angles are two angles that share a common vertex and side but have no common interior points if and only if two angles share a common vertex and side but have no common interior points, then they are adjacent angles.
4. Two angles are supplementary angles when the sum of their measures is 180° if and only if the sum of the measures of two angles is 180°, then they are supplementary angles.