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