The bilingual behavior which has provoked the most controversy in linguistics is undoubtedly intrasentential code-switching (CS). When two languages are to be used in a single sentence, various problems of incompatibility may arise. The most obvious derives from word-order differences – if a switch occurs at a boundary between two constituents which are ordered differently in the two languages, the resulting configuration will be ungrammatical by the standards of at least one. Another type of difficulty involves morphological disparity, as when a noun in one language must be inflected for case, where the other uses alternative means of accomplishing the same function. And there are many other problems having to do with subcategorization patterns, semantic differences, idiomatic constructions, etc.

It has been observed in systematic studies of bilingual communities that speakers tend to avoid these difficulties by eschewing switches at sites which would result in monolingually ungrammatical fragments. How is this accomplished? In earlier studies of Spanish–English bilingualism among Puerto Ricans in New York (Poplack, 1980, 1981; Sankoff and Poplack, 1981) we postulated the equivalence constraint, whereby switching is free to occur between any two sentence elements if they are normally ordered in the same way by the grammars of both languages involved, while prohibited elsewhere, as illustrated in.