The requirement for the rings of a [2]catenane to be oriented or facially dissymmetric for mechanical stereochemistry to arise is not a new observation; combining two oriented Cnh rings or two facially dissymmetric Cnv rings gives rise to the chiral catenanes originally identified by Wasserman and Frisch, 1 illustrated here using rings of C1h and C1v symmetry 15 respectively (Figure ). The vectors associated with the orientation or facial dissymmetry of the individual rings can never become coplanar in the catenane and thus the stereochemistry of such structures can be defined using the resulting oriented skew lines. The skew lines lie parallel to the associated ring when two oriented rings are combined but perpendicular to the rings when two facially dissymmetric rings are combined, which defines the canonical mechanically planar chiral (MPC) and mechanically axially chiral (MAC) stereogenic units of [2]catenanes, respectively. Thus, the only surprising result from our analysis is that S2n symmetric rings are oriented and thus give rise to a mechanical stereogenic unit, which to our knowledge has not previously been noted. However, we suggest that combining two S2n macrocycles (or a combination of S2n and Cnh rings) gives rise to the MPC stereogenic unit, as defined by the orientation of the skew lines associated with the rings, rather than a new form of mechanical stereochemistry. 17