Draw three circles in the plane, none of which lies completely inside another, and the common external tangent lines for each pair. Then points
of intersection of the three pairs of tangent lines lie on a straight line.
Monge's theorem has a three-dimensional analog which states that the apexes of the cones defined by four spheres,
taken two at a time, lie in a plane (when the cones
are drawn with the spheres on the same side of the apex;
Wells 1991).
