Points, also called polar reciprocals, which are transformed into each other through inversion about a given inversion circle C (or inversion sphere). The points P and P^' are inverse points with respect to the inversion circle if
OP·OP^'=OQ^2=k^2
(Wenninger 1983, p. 2). In this case, P^' is called the inversion pole and the line L through P and perpendicular to OP is called the polar. In the above figure, the quantity k^2 is called the circle power of the point P relative to the circle C.
Inverse points with respect to a triangle are generally understood to use the triangle's circumcircle as the inversion circle (Gallatly 1913).
The point P^' which is the inverse point of a given point P with respect to an inversion circle C may be constructed geometrically using a compass only (Coxeter 1969, p. 78; Courant and Robbins 1996, pp. 144-145).
Inverse points can also be taken with respect to an inversion sphere, which is a natural extension of geometric inversion from the plane to three-dimensional space.