---
title: "Treaties vs Customary Law and How Treaties Are Made"
description: "Treaties and custom rank as primary sources of international law; learn the VCLT 1969 treaty-making steps from adoption and signature to entry into force"
url: "https://modeldiplomat.com/learn/lessons/exam-international-law-treaties-vs-customary-law-how-treaties-are-made"
---

[New](https://modeldiplomat.com/)

Treaties vs. customary law; how treaties are made

Read 1 of 3

1/70 XP so far this lesson

Lesson 18 min 20 XP

# Treaties vs. customary law; how treaties are made

How treaties and custom rank as sources of international law, and the VCLT 1969 mechanics of treaty-making from adoption to entry into force.

## The Two Principal Sources

Article 38(1) of the Statute of the International Court of Justice enumerates the sources the Court applies: (a) international conventions (treaties), (b) international custom "as evidence of a general practice accepted as law," (c) general principles of law recognized by civilized nations, and (d) judicial decisions and the teachings of the most qualified publicists as subsidiary means. Treaties and custom are the two primary law-creating sources; the distinction between them is the spine of this lesson.

## Treaties: Consent in Writing

A treaty is an agreement concluded between States in written form and governed by international law (Vienna Convention on the Law of Treaties, 1969, Article 2(1)(a)). Treaties bind only the parties that consent to them — *pacta tertiis nec nocent nec prosunt*, codified in VCLT Articles 34–38: a treaty creates neither obligations nor rights for a third State without its consent. The governing maxim is *pacta sunt servanda* (VCLT Article 26): every treaty in force is binding upon the parties and must be performed in good faith. A party may not invoke its internal law to justify failure to perform (VCLT Article 27).

## Custom: Practice Plus Belief

Customary international law requires two elements. First, *general and consistent State practice* (the objective element). Second, *opinio juris sive necessitatis* — the belief that the practice is legally obligatory (the subjective element). The ICJ insisted on both in the *North Sea Continental Shelf* cases (Germany v. Denmark; Germany v. Netherlands, 1969), holding that even widespread practice does not crystallize into custom absent opinio juris, and that treaty rules can pass into custom if accompanied by such belief. The *Asylum* case (Colombia v. Peru, 1950) shows that practice must be constant and uniform. A State that objects clearly and consistently while a rule is forming — the *persistent objector* — may escape its application, as discussed in the *Anglo-Norwegian Fisheries* case (1951).

## How the Two Interact

Treaty and custom are not hierarchically ranked; they interact dynamically. A treaty may *codify* existing custom (much of the VCLT itself, and the 1958 Geneva Conventions on the Law of the Sea), *crystallize* an emerging rule, or *generate* new custom through subsequent widespread adoption (the *North Sea* test). Conversely, custom may modify treaty obligations over time. When norms conflict, *lex posterior derogat priori* (the later rule prevails between the same parties) and *lex specialis derogat generali* (the specific governs the general) guide resolution. Standing above both are peremptory norms — *jus cogens* (VCLT Articles 53 and 64) — such as the prohibitions of genocide, slavery, torture and aggression, from which no derogation is permitted and which void any conflicting treaty.

One question, then back to the lesson

Ask about this lesson