The enantiomers of axially chiral compounds are usually given the stereochemical labels (R) and (S), sometimes abbreviated (R) and (S).  The designations are based on the same Cahn–Ingold–Prelog priority rules used for tetrahedral stereocenters. The chiral axis is viewed end-on and the two "near" and two "far" substituents on the axial unit are ranked, but with the additional rule that the two near substituents have higher priority than the far ones.