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 the
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.