This contribution explores the professional and social relationships that takes place within the Supremes Courts. It proposes to take the French Conseil constitutionnel as an example. Classical literature on legal reasoning, especially in France, tries to explain decisions by referring to rational choice theory (economic analysis of law for instance). In contrast, I focus on the role played by all micro-practices such as role of clerks, documents attached to the decisions, old reports of deliberation, theories, and institutional procedures etc. What is the impact of such practices on the process of judicial decision making? According to this contribution, they influence much more than logical reasoning or external constraints such as political pressure.