The Vienna Convention on the Law of Treaties is the treaty about treaties, and it covers only written treaties between States

Background

The Vienna Convention on the Law of Treaties was concluded at Vienna, 23 May 1969, and the United Nations records that it is in force since 27 January 1980. The International Law Commission had placed the customary law of treaties among the topics suitable for codification at its first session, in 1949. By resolution 2166 (XXI) of 5 December 1966 the General Assembly endorsed the Commission's recommendation to convene an international conference, which met in two sessions at Vienna. Article 1 restricts the application of the Convention to written treaties between States, excluding treaties concluded by international organizations. Following an Advisory Opinion of the International Court of Justice, the Convention prohibits reservations which are incompatible with the object and purpose of the treaty to which they relate, under article 19 (c). The rule of interpretation in article 31 makes the object and purpose of a treaty, and its context, guidelines of interpretation. The Convention also addresses treaties that conflict with a peremptory norm of international law, known as jus cogens.