An exchange of letters (or exchange of notes) is a treaty in simplified form in which two governments record their agreement through paired correspondence: one party sends a letter proposing terms, and the other replies confirming acceptance, typically reproducing the original text verbatim. Under Article 13 of the Vienna Convention on the Law of Treaties (1969), consent to be bound may be expressed by exchange of instruments when the instruments themselves so provide or the parties otherwise agree. The form is legally binding under international law to the same extent as a formal treaty, provided the parties intend to create legal obligations.
The technique is favored for matters that are technical, urgent, or politically sensitive: aviation rights, visa arrangements, base access, financial assistance, or amendments to existing agreements. It avoids the elaborate signing ceremonies, full powers, and sometimes the parliamentary ratification associated with formal treaties — though domestic constitutional requirements still apply and vary by state. In the United Kingdom, exchanges of notes are routinely laid before Parliament under the Constitutional Reform and Governance Act 2010; in the United States, they are often concluded as executive agreements.
Drafting conventions are stylized. The initiating note opens with diplomatic courtesies, sets out the proposed terms in numbered paragraphs, and states that if the receiving government concurs, the note plus the reply shall constitute an agreement between the two Governments. The reply confirms agreement and usually specifies the entry-into-force date. Both letters are then registered with the UN Secretariat under Article 102 of the UN Charter.
Limitations: the form is bilateral and ill-suited to multilateral regimes, and the brevity that makes it attractive can leave interpretive gaps. Courts and tribunals have nonetheless treated such exchanges as fully binding, including the ICJ in Qatar v. Bahrain (1994), which held that the 1990 Doha minutes — though informally styled — created international obligations.
Example
In 2010, the United Kingdom and United States concluded an exchange of letters extending defense cooperation arrangements concerning the use of Diego Garcia.