Interpretation of Treaties

In one line

A treaty is interpreted in good faith according to the ordinary meaning of its terms in their context and in the light of its object and purpose, and preparatory work is a supplementary means, used to confirm that meaning or to resolve ambiguity or absurdity.

The general rule: article 31

Article 31(1). A treaty shall be interpreted in good faith in accordance with the ordinary meaning to be given to the terms of the treaty in their context and in the light of its object and purpose.

One rule, four elements. Good faith, ordinary meaning, context, object and purpose. The heading of article 31 is "General rule of interpretation" in the singular, and the International Law Commission chose the singular deliberately. The interpreter does not select textual, contextual or teleological interpretation as if choosing a school; the four elements work together in a single operation.

Good faith is not an independent licence to depart from the text. It is the same principle as in article 26, applied to reading rather than performing.

Ordinary meaning is the starting point, subject to article 31(4), which provides that a special meaning shall be given to a term if it is established that the parties so intended. The burden of establishing a special meaning lies on the party asserting it, which is what "if it is established" means.

What context includes: article 31(2)

The context comprises, in addition to the text, including its preamble and annexes:

(a) any agreement relating to the treaty made between all the parties in connection with its conclusion; and

(b) any instrument made by one or more parties in connection with the conclusion and accepted by the other parties as an instrument related to the treaty.

Two things follow. The preamble is part of the text, which is why preambles are the usual place to find the object and purpose stated. And a document is context only if all the parties made it, or made and accepted it as related; a unilateral statement nobody accepted is not context.

What is taken into account with the context: article 31(3)

(a) any subsequent agreement between the parties regarding interpretation or application;

(b) any subsequent practice in the application of the treaty which establishes the agreement of the parties regarding its interpretation;

(c) any relevant rules of international law applicable in the relations between the parties.

Subsequent practice is the most used and the most misused. Practice counts under 31(3)(b) only where it establishes the agreement of the parties. Practice by some States, unopposed but unshared, is not enough on its own, though it may sometimes be evidence of acquiescence.

Article 31(3)(c) is the article of systemic integration. It is the reason a treaty is not read in isolation from the rest of international law, and the reason a modern investment or trade treaty is read alongside human rights and environmental obligations binding on the same parties.