Geometry: Common Core (15th Edition)

When naming a segment, the order in which the endpoints are listed does not matter. If the two endpoints are the same, it is the same segment. $\overline{LR}$ and $\overline{RL}$ are the same segment. When naming an angle, the order in which the points on each ray are listed does not matter. The point that names the vertex is always the middle point in the name. The order of the other two points does not matter. If the vertex is the same, and the other two points are the same, it is the same angle. $\angle CBA$ and $\angle ABC$ are the same angle. Since the segment in the first statement is the same segment, and the angle in the second statement is the same angle, the Reflexive Property of Congruence applies.