The article is concerned with the question of how legal institutions are structured with the use of constitutive, institutive, consequential, and terminative rules. To that end, the regulation of international treaties as laid down in the Vienna Convention on the Law of Treaties of 1969 is analysed. This leads to the discovery of two additional categories of rules: content rules and invalidating rules. Finally, the special status of unique legal institutions is investigated. Unique legal institutions – for example, heads of state, parliaments, and supreme courts – enjoy validity in a legal system to the exclusion of the validity of any other legal institution of the same category in that system.