S2 and S1 are two extremes of a sliding scale of reactions. It is possible to find many reactions which exhibit both S2 and S1 character in their mechanisms. For instance, it is possible to get a contact ion pairs formed from an alkyl halide in which the ions are not fully separated. When these undergo substitution the stereochemistry will be inverted (as in S2) for many of the reacting molecules but a few may show retention of configuration. S2 reactions are more common than S1 reactions.