Focusing on the symmetry of the rings, we see that the stereogenic unit arises because the sole improper symmetry element of one ring (the mirror plane parallel to the macrocycle) cannot be made congruent with that of the other and so no improper symmetry elements remain in the catenane structure. Thus, the "topologically chiral" stereogenic unit of catenanes can be defined as arising when two Cnh rings (principal axis perpendicular to the ring plane) are interlocked. This analysis suggests that any structural feature that results in a Cnh ring will be suitable for the construction of a chiral catenane.