The model consists of a circular template, of radius a, hinged at O, a point on its circumference, to a bar OA, where OA = a. P is the centre of the circle, and E is the middle point of OP. G is a point on the circumference such that OP = OC = a, so that CE is perpendicular to OP.

From E two arms EX and EY radiate, and are no arranged by a linkage that the angles YEC and XEC are equal.