Let ray OA and ray OB be opposite rays in a plane. Ray OA, ray OB, and all rays with endpoint O that can be drawn on one side of line AB can be paired with the real numbers from 0 to 180 so that:
a. ray OA is paired with 0 and ray OB is paired with 180.
b. If ray OC is paired with x and ray ODis paired with y, then the measure of angle COD = |x-y|.