Detailed analysis of the symmetry properties of the mechanically axially chiral stereogenic unit of catenanes, and in particular the use of semi-structural representations, allowed us to identify an efficient co-conformational auxiliary approach to mechanically axially chiral catenanes and revealed a previously overlooked noncanonical axially chiral stereogenic unit in rotaxanes. The latter is a rare example of a "new" source of stereoisomerism, as opposed to an overlooked pathway of isomer exchange or an overlooked opportunity for atropisomerism , as have recently been reported. The rotaxane mechanical axial stereogenic is so closely related to that of catenanes it is surprising that it was overlooked for so long, which may in part be due to the use of schematic structures (Fig. ) that focus on symmetry without reference to underlying chemical structure; although these are useful they can also obscure important chemical information. Indeed, given that the fixed mechanical stereogenic units of catenanes (topological and axial) now both have an equivalent in rotaxane structures (planar and axial), it appears sensible to suggest that the stereochemistry of rotaxanes and catenanes be unified rather than treated as separate as they are typically . Our analysis also led to the surprising conclusion that catenanes based on two rings each containing a single stereogenic center can be described as either mechanically topologically or axially chiral but that only one mechanical stereodescriptor is required to specify their structure, an observation with implications for future studies.