Sector of Economy, Strength of Unions and the Opposition of Interest among Employers: The Case of the Flanking Measures Concerning the Treaty on the Free Movement of Persons’ The rules of the game in the sense of the legal framework are of major importance for negotiations between social partners. In Switzerland, these rules have changed with the adoption of the flanking measures to the free movement of persons’ treaty. One has, among other modifications, eased the conditions for declaring a collective wage agreement compulsory in the case of wage dumping. The employers associations had divergent preferences with regard to this modification. So far, one has explained this difference by the opposition between domestic and export oriented economy. However, the analysis of employers associations’ statements shows that the domestic economy did not unanimously accept the flanking measures. This contribution examines the divergence of interests inside the domestic economy and explains it by variations in the strength of the unions. Domestic sectors that are confronted with strong unions (for instance the construction sector), have an interest in facilitating the extension of compulsory wage agreements whereas domestic sectors with weak unions do not differ from the export oriented sector.