ABSTRACT Chaucer's use of the singular or plural form of the second person pronoun to address a single person in his Canterbury Tales usually follows the established standards of his time. However, some ninety instances of pronoun switching do occur, and explanations drawing on pragmatic parameters, rhyme and textual corruption have not been able to explain all of these deviations. Complementary to these approaches, this paper offers a novel explanatory hypothesis. The "collocational-phraseological hypothesis" suggested here takes into account the force of the syntagmatic relationship of words. On the basis of an original electronic compilation of all instances of pronoun switches in the Canterbury Tales and a classification according to three main types, we argue that frequently and/or habitually used lexical combinations (collocations, formulae, quotations) can account for a significant number of the cases in question.