It is when we turn to the MGI stereogenic unit of catenanes that we find a surprise. Because the two rings are distinct there are two possible products of the opening-andstoppering sequence, one of which is the canonical MGI rotaxane stereogenic unit identified by Schill, and the other is a previously overlooked form of rotaxane geometric isomerism. The former is characterized by the co-planar vectors associated with the two components lying parallel to the axle whereas in the latter these vectors lie perpendicular to the axle. We propose that the labels "type 1" and "type 2" are used to distinguish between the canonical and non-canonical geometric isomers of rotaxanes (MGI-1 and MGI-2 respectively), with the numeral assigned by the order in which they were identified.