In interwar France, the development of labor law and social insurances has led to a familiarization of the actors with the employment contract. It has given a larger scope to the labor conventions by enabling the integration of the isolated workers (working at home) in the population of the employees. Thus, labor law can be seen as an “investment in forms” defining labor above the split between industrial establishment and small firms industries. It explains why the vanguard of the movement was not the workers of the big rationalized plants, but the high skilled workers of the armament industries and the Parisian seamstresses. The negotiation addressed then firstly the classification of the working population in order to complete the ongoing unification of the wage earners.