This chapter examines the relationship between the prohibition of the use of force in article 2(4) of the UN Charter and customary international law: if they are identical, the role the customary rule plays in the interpretation of article 2(4) and which one to interpret or apply to determine the meaning of a prohibited ‘use of force’. In doing so it examines the following concepts: the use of pre-existing or subsequently developing custom to fill gaps in the treaty, the use of subsequently developing custom to informally modify the interpretation of the treaty, an evolutive interpretation of the UN Charter and informal treaty modification through subsequent practice. It argues that since article 2(4) is the origin of the customary prohibition, it is not appropriate to use pre-existing or subsequently developing customary international law to fill gaps in interpretation of article 2(4) nor to use subsequently developing custom to modify article 2(4). It concludes that due to the present relationship between the customary and Charter prohibitions, the preferable approach to determine the meaning of prohibited force under international law is to focus on interpreting the UN Charter.

This chapter tackles the issue of interpretation from the point of view of motion and time, mainly motion as change in space-time (kata topon kai chronon metavole), which depending on the interpretation could lead to an increase (auxesis), diminution (meiosis) of the treaty. This is mainly evidenced through the concepts of the principle of contemporaneity and evolutionary interpretation, and the chapter proceeds to demonstrate that the choice between these two is dependent on the ‘time-will’ of the parties. However, even evolutive interpretation has its limits and should never lead to an alteration (alloiosis) of the treaty. This chapter also analyses the rules of interpretation themselves, ie whether interpretative rules are constants, immutable and perennial ones, in the system of the law of treaties or whether they are, as any other rule, potentially susceptible to motion through time and change. This chapter demonstrates the false premise of the immutability of the rules of interpretation, and elaborates on the consequences of this to the choice between contemporaneous (static) and evolutive interpretation.