Linguistics, like many fields, likes to make use of convenient fictions which in literary criticism are called conceits. Diachrony and synchrony are such as we can never really slice up time into convenient units. Is collocation another conceit? Rigid definitions will always fail, as we are describing a natural phenomenon in which words form units. We can create a classification of phrasal units, but no classification can really reflect reality. At best we can talk in terms of prototypical collocations, and that is what I shall try and do.