Analysing the achiral building blocks of rotaxanes reveals the final mechanical stereogenic unit. The same analysis can be used to identify the axle point group symmetries that can give rise to mechanical stereochemistry in a rotaxane and thus the complete set of rotaxane stereogenic units (see ESI section 2). However, the same result is reached more intuitively by identifying that rotaxanes and catenanes are interconverted by a notional ring-openingand-stoppering operation (Figure ), which, as previously noted, leads to the conclusion that MPC catenanes and rotaxanes are directly related, as are the MAC pair. 5a Once again, these rotaxane stereogenic units can be differentiated by considering the relative orientation of the skew lines that characterise their configuration; the MPC stereogenic unit of rotaxanes is defined as arising when the vector associated with the axle lies along its axis, whereas the MAC stereogenic unit arises when the vector associated with the axle is perpendicular to its axis. These axle vectors lie perpendicular to the vector associated with the ring when interlocked with oriented or facially dissymmetric rings respectively.