This paper deals with the relationship between rules and conventionality, as reflected in a case study of the two structures N 1 de N 2 and N 1 du N 2. Most occurrences of these can be explained via two rules which evoke ease of referent identification and are a subset of the principles governing the ± definiteness opposition in French. Via dictionaries and Google searches, however, we detect variation that our rules cannot predict, typically when N 2 is non-countable and abstract. Local semantic contrasts, i.e. dependent on the lexical content of N 1 or N 2, further complicate the picture. We conclude that Coseriu's Norm, alias conventionality, must be recognized as a factor coexisting with the rules.