Custom on a Sliding Scale

Every student who has ever taken a traditional international law course has learned Manley Hudson’s four elements for the emergence of a rule of customary international law: (a) concordant practice by a number of States with reference to a type of situation falling within the domain of international relations; (b) continuation or repetition of the practice over a considerable period of time; (c) conception that the practice is required by, or consistent with, prevailing international law; and (d) general acquiescence in the practice by other States.