In the last fifteen years, a large number of studies have appeared in which specific cases of intra-sentential code-switching were analysed from a grammatical perspective, involving a variety of language pairs, social settings and speaker types. It was found that code-switching is a quite normal and widespread form of bilingual interaction, requiring a great deal of bilingual competence. In individual cases, intra-sentential code-switching is not distributed randomly in the sentence, but rather it occurs at specific points.

Where much less agreement was reached is with respect to general properties of the process. Various ‘constraints’ and ‘models’ regulating intra-sentential code-switching (the type most interesting from the grammatical perspective) have been proposed and tested, with the result that some cases appear to fall under one constraint, and others under another. This is by itself unsatisfactory. We do not know in any systematic way how different the models proposed are, neither intrinsically nor in their predictions. It should be mentioned at this point that many of the studies do not make the constraints or models very explicit, limiting themselves to descriptive statements. Therefore, an account is needed of the grammatical notions relevant to code-switching. These notions can then be used both to characterise specific instances of intra-sentential switching and to relate the various proposals in the literature to each other.