(a) it has signed the treaty or has exchanged instruments constituting the treaty subject to ratification, acceptance or approval, until it shall have made its intention clear not to become a party to the treaty; or (b) it has expressed its consent to be bound by the treaty, pending the entry into force of the treaty and provided that such entry into force is not unduly delayed.” Thus a State that has signed a treaty may not act in a way that defeats the object and purpose of a treaty. The importance of Article 18 has been underscored recently by the decision of the United States to “withdraw” from the Kyoto Protocol on Climate Change where it had signed but not yet ratified the agreement. This is a unique decision in international law, demonstrating that the United States is not prepared to accept obligations that might otherwise be applicable against it on the basis of signature alone. Aside from obligations that clearly exist at customary international law, States will typically deny that they have accepted any obligations unless and until they ratify the relevant treaty. Some treaties, such as the Vienna Convention, set out existing rules of custom as a codification of the law both for convenience and as a vehicle to progressively develop new additional rules. It is also recognised that a treaty can generate a new rule of custom. The International Court of Justice in the North Sea Continental Shelf cases (ICJ 1969, 3 at 42), accepted that a treaty provision could describe a customary rule provided that: • • the purported rule is of a potentially norm-creating character there has been a widespread and representative participation in the development of the treaty rule, particularly where the States whose interests have been especially affected, have agreed to that rule. In summary, treaties play the dominant role in contemporary international law both in formulating the obligations of the parties and in stimulating State practice that can crystallise as custom. 2. Custom Before the 20th century growth in multilateral treaties, international law consisted primarily of customary rules. Under the traditional theory of custom, the international customary rules apply to States on the basis of their express or implied consent. However, 31