Drag points A and B around the circle to set an arc, and drag point P, the inscribed vertex, anywhere else on the circle. The gold wedge at the center is the central angle that opens onto the highlighted arc between A and B; the blue wedge at P is the inscribed angle looking at that same arc. The central angle is always exactly twice the inscribed angle. When the arc is a semicircle, the inscribed angle is exactly 90 degrees — Thales' theorem. An info button opens a drawer explaining the theorem.