If two points A and A' are inverse with respect to a circle (the inversion circle), then the straight line through A' which is perpendicular to the line of the points A A' is called the polar of the point A with respect to the circle, and A is called the pole of the polar. An incidence-preserving transformation in which points and lines are transformed into their poles and polars is called a reciprocation.

diagonal triangle | inverse points | inversion | inversion circle | polar | polarity | reciprocal | reciprocation | trilinear polar | trilinear pole