Let be a line not passing through . Drop a perpendicular from to , meeting at the point (this is the point of that is closest to ). The image of under inversion with respect to is called the pole of . If a point is on a line (not passing through ) then the pole of lies on the polar of and vice versa. The incidence preserving process, in which points and lines are transformed into their polars and poles with respect to is called reciprocation.