The aim of this contribution is to reposition a key notion to the centre of Wulf Oesterreicher’s theoretical disciplinary reasoning: grammar, or more generally, ‘rules and norms’ specific to single languages and varieties the linguist deals with. This includes contingent, non-motivated structures, and claiming the necessity to analyse their existence ‘as such’ (i.e., also independently of communicative, social etc. factors) as the core task of linguistics. This point is illustrated with a much debated variable in French descriptive linguistics: past participle agreement. Combining theoretical and corpus linguistic observations, the distribution of the two variants (absence vs. realization of agreement) in constructions with the auxiliary avoir shows that a grammatical analysis interested in the derivation of certain rules and their application in actual language usage permits to understand the logic of linguistic variation - at least the one observable alongside the standard - non-standard opposition in French.