How do the verdicts of French joint industrial tribunals (Conseils de prud’hommes) participate in shaping the law in the field of labor relations? A “sociographic” survey brings to light both the multiplicity of forms of commitment to the institution and the key role played by a group of “professionals” in these labor tribunals, where elected representatives of employer associations and trade-unions sit side by side. Analyzing how councillors are socialized by the institution and how they arrive at their decisions shows that judgments stem from a combination of three different rationales: the law, union representation and the world of work. Between magistrates’ social characteristics, institutional affiliations and manners of judging, a new way of studying judiciary activity is suggested.