Statute of the ICJ, Article 38 The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply: a. international conventions, whether general or particular, establishing rules expressly recognized by the contesting states; b. international custom, as evidence of a general practice accepted as law; c. the general principles of law recognized by civilized nations; d. subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.
1969 Vienna Convention, Article 53 Treaties conflicting with a peremptory norm of general international law (“jus cogens”) A treaty is void if, at the time of its conclusion, it conflicts with a peremptory norm of general international law. For the purposes of the present Convention, a peremptory norm of general international law is a norm accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character.
Accordi di codificazione Esempi: Le 4 Convenzioni di Ginevra sul diritto del mare (1958), le due Convenzioni di Vienna sulle relazioni diplomatiche (1961) e sulle relazioni consolari (1963), la Convenzione di Vienna sul diritto dei trattati (1969) e la Convenzione di Vienna sulla successione degli Stati nei trattati (1978), il Progetto di articoli sulla responsabilità internazionale degli Stati, la Convenzione di Montego Bay sul diritto del mare (1982).