Coalition agreements are often presented as the principal coalition governance instrument. According to scholars, they have three functions: a communicative function, a policy-making function and a conflict resolution function. Starting from a critical discussion of these researches, this article aims at elucidating the conditions and the modalities of their mobilization by coalition members. It is based on an ethnographic study of the Left Union ruling coalition of the French city of Calais from 1971 to 2008. The analysis of the contents and the multiple uses of these contracts show that they constitute both the frameworks and the stakes of the coalition negotiations. Their presentation in the form of formal, written contracts can be explained by the properties given by the parties to this juridical form, namely their capacity to reduce the uncertainties and risks inherent in the coalitional association while preserving the room for maneuver in political negotiations and competition. Thus, they are marked by a constitutive tension, between the game from which they issue and their official objective, that of the regulation of coalition relationships.