explementary

The explementary arc of an arc $a$ of a circle is the arc forming together with $a$ the full circle.

Two angles are called explementary angles of each other, if their sum is the full angle $2\pi$, i.e. ${360}^{\circ }$. In the below picture, the  $\alpha ={60}^{\circ }$  of an equilateral triangle and its explementary angle  $\beta ={300}^{\circ }$  (which is an of the triangle) are seen.