The Convention codifies several foundations of contemporary international law. It defines a treaty as “an international agreement concluded in writing between States and subject to international law” and reaffirms that “each State has the capacity to conclude treaties”. Article I limits the application of the Convention to written treaties between States and excludes treaties concluded between States and international organizations or international organizations themselves. Article 26 defines pacta sunt servanda, article 53 proclaims the peremptory norm and article 62 proclaims the fundamental change of circumstances. The Vienna Convention on the Law of Treaties (VCLT) is an international agreement that governs treaties between States.  Known as the “contract on contracts”, it defines detailed rules, procedures and guidelines for the definition, design, modification, interpretation and generalization of the implementation of contracts.  The VCLT is considered to codify customary international law and state practice with respect to treaties.  However, agreements between States and international organizations or between international organizations themselves are governed by the 1986 Vienna Convention on the Law of Treaties between States and International Organizations or between International Organizations when it enters into force. In addition, the provisions of the Convention among the Members of the State continue to apply in treaties between States and international organizations.  The Convention does not apply to un written agreements.  The Convention applies only to treaties concluded after its creation and to those concluded between States, and therefore does not regulate agreements between States and international organizations or between international organizations themselves, but if one of its rules is legally binding on these organizations, they remain so.  The VCLT applies to contracts between States within an intergovernmental organization.
 The principles of interpretation codified in section 31 are used prior to the application of the principles of interpretation set out in section 32, which explicitly states that it offers complementary interpretations. This Convention shall be open for signature by all States Members of the United Nations or of one of the specialized agencies or of the International Atomic Energy Agency or of the Parties to the Statute of the International Court of Justice, as well as by any other State invited by the General Assembly of the United Nations to become a party to the Convention; as follows: until 30 November 1969 at the Federal Foreign Office of the Republic of Austria, then until 30 April 1970 at the United Nations Headquarters in New York. The VCLT is considered one of the main instruments of contract law and remains a relevant guide in the event of a dispute relating to the interpretation of contracts.  When a “State” treaty is opened, it may be difficult or impossible for the depositary to determine which States are. If the Treaty is limited to the members of the United Nations or to the parties to the Statute of the International Court of Justice, there is no ambiguity. However, a difficulty has arisen with regard to the possibility of participating in treaties when entities that otherwise appeared to be States are not admitted to the UN or have not applied to become members of the ICJ or the UNITED Nations due to the opposition of a permanent member of the Security Council. Since that difficulty had not been encountered with regard to membership of the specialized agencies that were not subject to a veto procedure, some of them had become members of specialized agencies and had therefore been recognized as States primarily by the international community. In order to allow for the widest possible participation, a number of conventions provided that they were also open to Member States of specialized agencies.
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