International law is affected by positive (written) law, founded on usage, consent, and agreement.

Explanation

(English-origin maxim.) The rules governing relations among nations derive much of their operative force from established practice, usage, and express agreement (treaties and conventions). This maxim describes the mechanism by which international law functions in practice — it does not diminish the supremacy of natural law, which remains the foundation against which all positive law, domestic or international, is measured.

Where this applies

• Two countries have traded under an informal custom for decades; when a dispute arises, the court looks to the established usage and prior agreements to determine their obligations.
• A maritime dispute between nations is resolved by reference to centuries of seafaring custom, formal treaties, and the consent of the parties — not abstract theory alone.
• An international trade body settles a tariff dispute by examining the written agreements between member states and the longstanding commercial practices both nations have followed.