This article looks into the origin of the uses of arrière- and après- as formatives, as well as into the way they have been grammaticalized, for instance in arrière-cour ‘backyard’ and après-match ‘after match’. The analysis of the functioning of these two terms in Old French shows that they both were multi-categorial, since they could be used as a preposition, a conjunction or a particle, but that they each also had a preferential use : après was fundamentally a preposition, whereas arrière was rather used as an adverb or a particle. The paper then shows that in the case of après, it was its prepositional use that allowed it to emerge as a formative, whereas the use of arrière as a formative resulted from a substitution of the ancient prefix r(i)ere-. The paper furthermore raises the question of the degree to which both elements have been grammaticalized, and thus also of their status : have they turned into true prefixes in Modern French or should they still be conceived of as prefixoïds ?