Our results confirm that catenane 5 exists as a pair of enantiomers whose stereochemistry results from the mechanical bond and whose configuration is defined by the exocyclic double bond. This is consistent with our proposal that this molecule contains a mechanical stereogenic unit analogous to previously reported catenanes labelled simply as "topologically chiral", but it should also be clear that in this case the stereochemistry is not topological nature, as confirmed by the racemization of 5 in solution through double bond isomerization. Given that the highest symmetry representations of both rings of catenane 5 are Cnh (n = 1), is the case with all previously reported topologically chiral catenanes, we do not think it is reasonable to consider catenane 5 as a new class of chiral mechanically interlocked molecule. It is much simpler to acknowledge that there is a single catenane stereogenic unit when two oriented Cnh rings are combined but that, depending on the structure of the rings, the stereochemistry can be either topological or Euclidean (Fig. vs Fig. ). This allows us to unify the stereochemistry of rotaxanes and catenanes composed of oriented components. The stereochemistry of rotaxanes (which are topologically trivial) and catenanes (which are not) is often considered independently but is clearly related; the schematic representation of a topologically chiral catenane can be converted to that of a mechanically planar chiral rotaxane by the notional process of ring opening and stoppering (Fig. ). Given that we have demonstrated that the "topological" stereogenic unit of is not inherently topological in nature, we propose that their relationship could be made clearer if both stereogenic units were united under the same name, mechanically planar chiral. In the case of catenanes, some examples will be topologically mechanically planar chiral (all previously reported examples), and others will simply be mechanically planar chiral (catenane 5).