Article 13 Vienna Convention on the Law of Treaties

Consent to be bound by a treaty expressed by an exchange of instruments constituting a treaty

The consent of States to be bound by a treaty constituted by instruments exchanged between them is expressed by that exchange when:

(a) the instruments provide that their exchange shall have that effect; or

(b) it is otherwise established that those States were agreed that the exchange of instruments should have that effect.

See also: Article 12 Vienna Convention on the Law of Treaties