Articles 31 and 32 of the 1969 Vienna Convention on the Law of Treaties (VCLT) codify the customary international law rules governing how treaties are interpreted, and are routinely invoked by the International Court of Justice, WTO panels, investment tribunals, and human rights bodies.
Article 31 — General rule. A treaty must be interpreted in good faith in accordance with the ordinary meaning of its terms in their context and in the light of its object and purpose. Context includes the treaty's text, preamble, and annexes, plus related agreements made between the parties in connection with the treaty. Interpreters must also take into account subsequent agreements between the parties regarding interpretation, subsequent practice establishing such agreement, and any relevant rules of international law applicable between the parties. A special meaning attaches to a term only if the parties so intended.
Article 32 — Supplementary means. Recourse may be had to supplementary means of interpretation, including the preparatory work of the treaty (travaux préparatoires) and the circumstances of its conclusion, either to confirm the meaning resulting from Article 31, or to determine meaning when the Article 31 interpretation leaves the meaning ambiguous or obscure, or leads to a result that is manifestly absurd or unreasonable.
The relationship between the two articles is hierarchical: Article 31 is the primary, holistic interpretive exercise, while Article 32 plays a confirmatory or curative role. This structure deliberately downgrades pure intentionalism (heavy reliance on negotiating history) in favor of a textual-contextual-teleological approach. Tribunals such as the WTO Appellate Body in US – Gasoline (1996) have repeatedly affirmed that Articles 31–32 reflect customary international law and thus bind even states not party to the VCLT itself.
Example
In the 2009 *Dispute Regarding Navigational and Related Rights* (Costa Rica v. Nicaragua), the ICJ applied VCLT Article 31 to hold that the term 'comercio' in an 1858 treaty should be interpreted with its evolved contemporary meaning.