This paper deals with the relationship between rules and conventionality, as reflected in a case study of the two structures N1de N2 and N1du N2. 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 N2 is non-countable and abstract. Local semantic contrasts, i.e. dependent on the lexical content of N1 or N2, further complicate the picture. We conclude that Coseriu's Norm, alias conventionality, must be recognized as a factor coexisting with the rules.