The assignment of the mechanically axially chiral stereogenic unit relies on identifying the highest priority faces of each ring, as proposed by Stoddart and Bruns 7 (Supplementary section 13.1). However, because this rule had not been applied in a real system, we immediately encountered difficulties; to unambiguously assign the highest priority face of each ring the relative orientations of the prochiral units must be specified. On reflection, we suggest that in the case of catenanes the inplane substituents of the prochiral moieties be oriented so they "point" towards one another (Fig. ).