If all ligands are finally distinguished after application of Rule 5, an additional test should be done to count the number of pairs of enantiomorphic ligands. The final descriptor will be r/s, m/p, or, in the case of akenes, seqCis/seqTrans, if and only if this number is one (Figure ), otherwise it will be R/S, M/P, or seqcis/seqtrans (Z/E). In terms of implementation, the criterion for pseudoasymmetry at a tetrahedral center is that, when comparing otherwise identical ligands, there is an odd number of pairs that reverses priority when comparing like/unlike sequences using an S reference vs. using an R reference. So, for example, in Figure , the right-hand R ligand has higher priority with the R-reference, but the lefthand S ligand has higher priority with the S-reference. The priority switches, and we have the normal outcome for Rule 5pseudoasymmetric. But in Figure , priority switches twice, so the result is asymmetric.