Conversely, in the equivalent rotaxane, we suggest they be oriented to point in the same direction (Fig. ). The latter, somewhat counterintuitive, proposal is designed to ensure that a mechanically axially chiral rotaxane derived from the notional ring opening of an axially chiral catenane would retain the same stereolabel. The absolute stereochemistry of both mechanically axially chiral catenanes and rotaxanes can then be assigned by viewing the ensemble along the axis connecting the prochiral units and observing the relative orientation of the vectors from the out of plane substituent with the highest priority to the lowest priority as shown; a clockwise direction of rotation from the head of the front vector to the tail of the rear vector is assigned as Rma and an anticlockwise path assigned as Sma. This approach can be readily extended to molecules where facial dissymmetry arises due to stereogenic axes or planes (Supplementary section 13.2).