Use the slider to change the locus. Drag point C around the circular arc and observe the congruency of the two angles.
Given AB and an angle θ, the locus is the set of all points C such that the angle ACB is θ. The angle between d and AB is θ; c is the right bisector of AB. P is the intersection of c and e, a perpendicular to d through point A. The slider value is in radians.