A stereoheterotopic group c (as in tetrahedral Xabc₂) is described as pro-R if, when it is arbitrarily assigned CIP priority over the other stereoheterotopic group c, the configuration of the thus generated chiral centre is assigned the stereodescriptor R. The other group c is then described as pro-S. This method for distinguishing between stereoheterotopic groups can be applied to other kinds of prochiral molecular entities or prochiral parts of molecular entities considered on their own.