Most metropolitan institutions in France are governed by grand coalitions, in which the main left and right political parties are allied. Politicians involved in these unusual (for France) alliances point to the alleged peculiarities of these institutions to justify them. This paper, based on ethnographic fieldwork, shows how these grand coalitions, far from being the result of a natural agreement on the goals of municipal policies, is the result of structural compromises between the mayors in order to avoid partisan conflicts and to share policy resources at this institutional level. The paper also explores some of the specific organizational mechanisms which support relationships between the members of these coalitions.