In this contribution, an endogenous conception of law is defended which can be deployed in regard of different scales of time and domains of the law. In this article, the focus is mainly on labor law. In the first part, a history of the French employment contract law is presented by proposing a grid of its foundations and evolution from the distinction of different principles of justice generating conventions. The change of conventions and its impact on law is precisely treated. In a second part, the recent evolution of this law will be exposed by showing the key role played by legal intermediaries in the contractualization of the working relationship, based on a database of employment contracts. It is shown how lawyers can generate an increasing strategic use of law, disconnected from territorialized conventions, if they do not play their role of mediator between different logics. In the conclusion, the emergence of a transnational law and the transformation of the lawyer profession are questioned.