Two different molecules
Xabcd and Xabce, may be said to have the same relative configurations if e takes the
position of d in the tetrahedral arrangement of ligands
around X (i.e. the pyramidal fragments Xabc are superposable). By the same token
of Xabce may be said to have the opposite relative configuration to Xabcd. The terms
may be applied to
molecular entities with central atoms other than carbon but are limited to cases
where the two related molecules differ in a single ligand.