Based on a geopolitical analysis of the changes to the perimeter of public local authority intervention, with a case study of SYTRAL (the authority organizing public transport in the Lyon agglomeration), this paper discusses two presuppositions behind recent developments in inter-municipal co-operation. First, when incorporated as districts (or syndicats), it is supposed that an authority would justify its actions through technical and engineering logics, leaving political stakes undiscussed. Contrary to this supposition, this article underlines the importance of political stakes behind the definition of SYTRAL’s perimeter of intervention. The definition of this perimeter involves factors and arguments around inter-municipal relations, local economic development policies, urban planning, transport, territorial identity and management of population distribution. The second point, discussed in the conclusion of the article, is that, after years of reforms which have sought to marginalize them, syndicats still appear relevant for inter-municipal cooperation. They offer the possibility of taking advantage of the fine-grained French municipal fabric to fine-tune intervention perimeters to local needs and to develop inter-territorial cooperation.