The conversion of a @[email protected] into a pure @[email protected] or into a mixture in which one @[email protected] is present in excess, or of a @[email protected] mixture into a single diastereoisomer or into a mixture in which one diastereoisomer predominates. This is sometimes called deracemization. If the two enantiomers of a @[email protected] substrate A are freely interconvertible and if an equal amount or excess of a non-racemizing second enantiomerically pure chemical species, say (R)-B, is added to a solution of @[email protected] A, then the resulting equilibrium mixture of adducts A·B will, in general, contain unequal amounts of the @[email protected] (R)-A·(R)-B and (S)-A·(R)-B. The result of this @[email protected] is called @[email protected] @[email protected] of the first kind. If, in such a system, the two diastereoisomeric adducts differ considerably in @[email protected] so that only one of them, say (R)-A·(R)-B, crystallizes from the solution, then the @[email protected] of @[email protected] in solution and concurrent @[email protected] will continue so that all (or most) of the substrate A can be isolated as the crystalline diastereoisomer (R)-A·(R)-B. Such a '@[email protected] @[email protected] @[email protected]' is called an @[email protected] @[email protected] of the second kind.