This paper aims to contribute to elucidating the notion of congruence in code-switching with particular reference to Welsh-English data. It has been suggested that a sufficient degree of congruence or equivalence between the constituents of one language and another is necessary in order for code-switching to take place. We shall distinguish between paradigmatic and syntagmatic congruence in relation to the grammatical categories of the two languages, focusing on the insertion of English nouns and noun phrases, adjectives, verbs and participles in otherwise Welsh utterances. We shall demonstrate how differing degrees of congruence between grammatical categories in the two languages are reflected in different code-switching outcomes.