In this article I identify and analyse the legal instruments that allow international courts to contribute to the development of international law. I explain how these contributions are linked to a process that I call autonomisation, and by which I understand that international courts form and sustain an autonomous interpretation of what the law is within their jurisdiction. The aim of this analysis is to be able to identify the legal format that international courts use to respond doctrinally to the overall socio-political context of the legal disputes that it aims at settling through its decisions. It is, I claim, in this cross-field between law and politics that courts unfold a specific form of diplomacy: legal diplomacy.