Let t be the centre of a circle, radius r, p any point in its plane, and draw the diameter through p. Then the inverse of p is a point p′ on tp such that tp. tp′ =r2. This circle is called the circle of inversion, its centre t the pole of inversion and the square of its radius the constant of inversion, and each of the points p, p′ is the inverse of the other. This relation is a metric one: a descriptive relation can be obtained as follows.