Three versions of the principle of proportionality exist in international law, with different constitutional functions. The horizontal version 1 applies, inter alia, in the field of countermeasures. The principle here refers to the relation between action (breach of international law) of the state and the admissible reaction of another state. The diagonal version 2 concerns the relation between and national public interest and particular interests, of individuals (for example, in international humanitarian law), or of investors. The vertical version 3 refers to the relation between a global public interest, for example, in free trade, and particular interests of states.
This chapter shows that the constitutionalization of and within international law is a fragmented process which, moreover, engages domestic constitutional law. It is not bringing about a ‘superconstitution’ over and above domestic law and all international subfields. After clarifying the key terms, notably constitutionalization, constitutionalism, and constitutional law, it explains the sectoral constitutionalization of various international organizations and the constitutionalization of the private (economic) realm. It concludes that we find (only) constitutional fragments.