What a Treaty Is, and Its Kinds

In one line

A treaty is a written agreement between States, intended to create legal obligations and governed by international law, and its name makes no difference at all.

In the wording a student can write in an exam: article 2(1)(a) of the Vienna Convention on the Law of Treaties 1969 defines a treaty as an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation. The essentials are therefore an agreement, between States, in writing, and governed by international law.

The Convention, and what it does not reach

Article 1. The present Convention applies to treaties between States.

That limit matters. Agreements between States and international organisations, and between organisations, are covered by a separate convention of 1986 and not by this one.

Article 3 saves everything the Convention does not reach: the fact that it does not apply to international agreements concluded between States and other subjects of international law, or between such other subjects, or to agreements not in written form, does not affect the legal force of such agreements, the application to them of any rules in the Convention to which they would be subject under international law independently of the Convention, or the application of the Convention to relations between States under agreements to which other subjects are also parties.

Read article 3 carefully, because it answers the commonest objection to the definition. An oral agreement between States is not covered by the Convention and is not thereby invalid: it is a treaty at customary law, and much of the Convention applies to it as custom.

Article 4, non-retroactivity. Without prejudice to the application of any rules set out in the Convention to which treaties would be subject under international law independently of it, the Convention applies only to treaties concluded by States after its entry into force with regard to those States. So the Convention is not retrospective, and older treaties are governed by the customary rules, most of which the Convention states.

Article 5 applies the Convention to any treaty which is the constituent instrument of an international organisation, and to any treaty adopted within an international organisation, without prejudice to the relevant rules of the organisation. So the Charter of the United Nations is itself a treaty, and the rules in this module apply to it.

Article 6. Every State possesses capacity to conclude treaties. That is short and it is the point of principle: treaty-making capacity is an incident of statehood.

The definition, element by element

Article 2(1)(a). A treaty means an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments, and whatever its particular designation.