Okay, so basically have the outside of a circle. That arc has some measurement in degrees (or radians, if you’ve done those). If you inscribe an angle from the endpoints of the arc to the center of the circle, the angle created at the center will be the same angle as the measure of the arc. However, if you do it to any point on the circle (that is not in the arc), then its measure is half of the measure of the arc.