Words that are regularly used together in vernacular speech, such as ‘clean and tidy’, or ‘neat as a pin’. Interrupting the regular pairing of words is an important, albeit easy way of achieving poetic ressentiment---for example, instead of saying ‘I’m afraid I cannot do that’, one might also write, ‘I’m scared that I cannot do that’. The collocation in the second case is completely wrong and is typical of the kind of error a non-native speaker might make. The point is that there is no logical reason why one should say ‘afraid’ rather than ‘scared’ in this context, but the habit of national speech dictates that the former is right and the latter is wrong.