Certain Rules of Customary Law

Doctrine International Law Preferred

Particular rules of customary international law identified by general State practice accepted as law.

Independent referenceNot an official or legal definition
Sources citedSee the sources at the end of this page
Status Under reviewLast updated 11 October 2026
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Definition

In plain language

Some rules of international law are not written in treaties but arise from what States consistently do because they believe the law requires it. This term refers to particular rules of that kind.

Academic definition

Customary rules are identified by two elements, a general practice and acceptance of that practice as law (opinio juris), as set out in Article 38(1)(b) of the ICJ Statute and applied in North Sea Continental Shelf (1969) and Nicaragua (1986). The ILC's 2018 Conclusions on the identification of customary international law confirm the two-element approach and note that States' practice and statements are the main evidence. Whether a given rule meets those tests is often disputed, for example for the prohibition of the use of force or for particular rules of immunity.

Classification

Term type
Doctrine
Category
International Law
Subject field
International Law
Subfield
Sources of International Law
Difficulty level
Advanced
Grammatical category
noun phrase

Language variants

Alternative terms
  • identifiable rules of customary international law
  • specific customary rules
Non-preferred terms
  • not_applicable
Abbreviations
  • not_applicable
Acronyms
  • not_applicable
Native terms
  • not_applicable
Transliteration
not_applicable
Pronunciation
not_applicable

Context

Legal effect: Binding

Scope

Covers specific rules whose customary status is asserted or tested; the general doctrine of custom is treated in related records, and treaty rules are different even when they reflect custom.

Legal context

ICJ Statute Article 38(1)(b) lists international custom as a source; the ILC Conclusions of 2018 describe identification; the ICJ in Nicaragua held that custom and treaty rules can exist side by side with the same content.

Diplomatic context

States argue that rules are customary to bind non-parties to a treaty or to resist new claims, and persistent objectors may claim exemption.

Institutional context

Identified by the ICJ, arbitral tribunals, the ILC, national courts and the ICRC, whose 2005 study identified customary rules of humanitarian law.

Historical context

The doctrine of custom dates back centuries, while the modern test was shaped by the PCIJ and ICJ and refined by the ILC in 2018.

Why it matters

A rule recognised as custom binds every State, which matters most where a State has not joined the relevant treaty.

Usage

Register: Legal

Usage note

The title is unusual, so editors should check whether it is meant as a heading for specific rules or as a mistaken title.

Example

The court held that the rule had become part of customary international law through consistent State practice and opinio juris.

Example scenario

A court is asked to apply a rule from a treaty that State A has not ratified. State B argues the rule is customary and shows official statements and consistent practice by many States, while State A claims it objected from the start.

Common misuse

Treating a repeated practice by a few States as custom, without proof that it is general and accepted as law.

Not to be confused with

  • General principles of law
  • Treaty law
  • Jus cogens

Sources

  1. Statute of the International Court of Justice, International Court of Justice, Article 38(1)(b). icj-cij.org
  2. Official definition. not_found_in_source
  3. Treaty reference. icj-cij.org
  4. International Law Commission, Draft Conclusions on Identification of Customary International Law (2018). legal.un.org
  5. not_found_in_source. not_found_in_source

Source type: Treaty. Source reliability: Primary.

This entry is an independent reference definition, not an official or legally binding text. Where an official definition exists, it is linked above and is authoritative. Read our methodology.

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