It is the locus (line r) of points from which equal tangents to the two circles {a,b} can be drawn. It coincides with the locus of centers of circles simultaneously orthogonal to two given circles. Concentric circles have no real radical axis (their radical axis coincides with the line at infinity). Every circle (d) intersecting the two circles defines common chords ad, bd which intersect on the radical axis of {a,b}. The common point C is actually the [Radical Center] of the three circles.