Complete decreolization, when mergers relative to the standard language have occurred in the historical basilect, assumes an impossible form of sound change: unmerger. This section discusses the irreversibility of merger in detail, and then discusses the consequences of this principle for the view of the development of Jamaican Creole and other ``decreolized'' languages. If two sound classes are merged in lower strata, but not in higher strata of a creole community, then those social levels that retain the distinction, must have existed from the earliest time in the development of the creole. The arguments presented here support recent arguments for social and linguistic diversity in the development of ``decreolizing'' creoles (Patrick 1991).