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Treaties and Customs as a Source of International Law

“No man is an island.”
(John Donne)

The global interconnectedness of the 21st century is far more than a stone’s throw away from when man and his habitus was truly an island—at the beginning of time as we remember it. The phenomenon of international law, as the body of legal rules, norms, and standards that apply between sovereign states and other entities that are legally recognized as international actors[1], is a testament to this very fact. The sheer interdependence and reliance on which nations are built and thrive provide the basis for the sustenance of international law. However, this body of law is hardly a static entity. To the contrary, it is very much vital and dynamic in that it continues to adapt and cater to the needs of society as we progress through a series of cultural and moral evolutions. Needless to say, the sources of creation of international laws must necessarily keep pace with the aforementioned societal evolutions. Herein, it becomes pertinent to dissect and discuss the relevant sources and their gravity in governance.


Sources of International Law

Although, there is no formal literature that explicitly lists the sources of international law per se, the statute of the International Court of Justice does identify certain categories as the basis for deciding international disputes. It is to this extent that Article 38 of the statute of the International Court of Justice[2] identifies the sources of international law and states that,

  1. The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:
  1. international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
  2. international custom, as evidence of a general practice accepted as law;
  3. the general principles of law recognized by civilized nations;
  4. 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.

Hence, the sources of international law may be summarised as international treaties and conventions, international customary law, the general principles of natural law as recognized by nations, and judicial decisions and teachings of authority.

Hierarchy of Sources

The hierarchy of preference of sources may be simplified to say that the primary sources of international law are international treaties and conventions and international customary law, supplemented by the principles of natural law and auxiliary judicial decisions and teachings of authority. However, this is further supplemented by other principles such as that specific rules take precedence over general rules; and that recent decisions supersede old ones. Nevertheless, it is generally accepted that international treaties and conventions as well as customary law are of prime importance and gravity in the formulation of international law. Furthermore, these are the two pertinent sources within the scope of discussion hereinafter.


International Treaties and Conventions

According to Article 2 of the Vienna Convention on the Law of Treaties which outlines the role of international treaties and conventions as a source of international law, 'treaty' means an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation.[3]

Basis of Treaties

Treaties are generally considered as persuasive law that confers obligations on parties to it. However, the question of whether a treaty is ipso facto law or merely a legal obligation is a distinct debate altogether. Regardless, neither perspective takes away from the fact that treaties are based on the time-tested and enduring principle of ‘pactasuntservanda’ or ‘agreements must be kept’.It is this very principle that demands obedience to treaties ratified by nations and essentially forms the indispensable foundation of a just system governing sovereign nations as equals.

Nature of Treaties

The question of whether a treaty is law or a set of contractual obligations amongst sovereign nations leads us to the dichotomy in the nature of treaties. Thus, treaties may be of two kinds—

  1. Law-making Treaties: These are multilateral treaties and mainly intend to have universal or general relevance.
  2. Treaty-Contracts: These treaties apply only between two or small number of states. They are mainly bilateral treaties.[4]

Inspite of the existence of this classification, there is not much effectual distinction made by the difference in nature of treaties. Both contractual as well as obligations in the form of law have authority and nations agree to be bound by them. Hence, both bilateral and multilateral treaties essentially affect countries that are party to it in the same way—they are persuasive and binding in nature.

For the sake of clarity, it may be said that multilateral treaties conferring legal obligations that a multiplicity of nations agree to be legally bound by confer the authority of international law upon the treaty in effect whereas, bilateral treaties or such treaties as those that have a narrower scope of application generally bear closer resemblance to contracts conferring contractual obligations on nations rather than international law as we speak of it; although in reality, they bind the behaviour of nations in nearly the same way.

Mechanisms governing Treaties

The Vienna Convention on the Law of Treaties is the principal instrument detailing the mechanisms that govern treaties. Thus, treaties too are subject to certain rules that decide their application and authority.

Firstly, treaties are consent-based inthe sense that states voluntarily agree to be bound by the terms of a certain treaty; that is, they consent to a treaty. A state is only governed by a treaty if it has ratified it, though there are exceptions to this. For example,delimitation of territorial boundaries is applicable on all states who must respect it, i.e. it is ‘ergo omnes' or ‘against the whole world’.The means of expressing consent is dealt with in Article 11 of the aforementioned Vienna Convention, which states that ‘the consent of a State to be bound by a treaty may be expressed by signature, exchange of instruments constituting a treaty, ratification, acceptance, approval or accession, or by any other means if so agreed’.[5]

Reservations, Declarations, and Derogations

The purpose of a treaty is to encourage countries to be signatories to it and increase its universality of application so as to hold countries to the same standard of justice. However, the process of drafting an international treaty is cumbersome and drawn out owing to the number of states that are involved in the process and the resultant multiplicity of viewpoints. The process of drafting a treaty includes stark disagreements and debates upon every aspect of the agreement. In order to increase the number of signatories and ratifications of a treaty, and hence global order, international law allows a system of picking and choosing for nations wherein they may limit application of certain provisions of the agreement under consideration. This is done through reservations, declarations and derogations.[6]

Reservationsare defined by the Vienna Convention as:
‘A unilateral statement, however phrased or named, made by a State, when signing, ratifying, accepting, approving or acceding to a treaty, whereby it purports to exclude or to modify the legal effect of certain provisions of the treaty in their application to that State.[7]

Only specified reservations are permitted and they cannot undermine the object and purpose of the Treaty.

Declarations, unlike reservations, do not affect legal obligations, but are often made when a State expresses its consent to be bound by a specific treaty. The State uses the declaration to explain or clarify its understanding of particular aspects of the treaty text.  

Derogations allow for a system where countries can make space for certain situations and exigent circumstances whereupon the effect of the provisions of a treaty may be suspended or otherwise derogated. For instance, in a situation of war or internal conflict, the right to assemble peacefully may be suspended. However, some basic human rights cannot be derogated from under any circumstances, notably the prohibition on torture, inhumane and degrading treatment. This ensures basic protection of rights.


International Customary Laws

The statute of International Court of Justice refers to “international custom, as evidence of a general practice accepted as law,” as another formal source of international law. However, any general practice is not termed as customary law and certain essential criteria is present which must be satisfied in order for a custom to attain recognition as international customary law.


For a custom to be authoritative as law, it must fulfil two essential conditions.

  1. State practice

The actual practice of states includes multifarious elements such as duration, consistency, repetition, and generality of a particular kind of behaviour by states. For any custom to become law, it is essential that continuous or established usage be established for a significant time period. This requirement is termed as ‘usus’.

Notably, however, in the North Sea Continental Shelf cases in 1969[8] ,the ICJ expressly accepted the possibility that a wide spread and representative practice could generate a rule of customary international law even without the passage of any considerable period of time. This is referred to as instant customary law.

  1. Acceptance of legal nature of practice

This is also referred to as ‘opinio juris’ and is rooted in the principle that for a custom to be accepted as law, it is not merely enough that a practice has long and established usage. Such usage must also possess, in addition, the recognition of the practice as legal and not merely a matter of policy by states. As was pointed out in the North Sea Continental Shelf Cases[9], ‘Not only must the acts concerned amount to a settled practice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it.’


Once a custom becomes international law, all states must comply with it irrespective of whether they have explicitly consented to it or not. The only exception that may be made is if a certain state has objected to the custom or usage from its very inception. Evidence of such objection must then be demonstrated and this evidence must be of sufficiently weighty nature. Thus, states are obliged to behave in accordance with international customary law; there is no choice to comply as in the case of treaties or conventions. This may be observed to be the primary advantage of customary law over treaty law.



From the above discussion, it is clear that neither source—treaty or custom—is exhaustive in the creation of international law. Each is fraught with its own flaws where treaties are dependent on ratification by countries and customs are slow in birth and evolution. Moreover, customary law is not as easy to establish and requires widespread practice and acceptance as well. However, a synthesis of treaties and customs greatly reduces the drawbacks of each standalone source and hence results in an enhanced source of law that is far more effectual than its components. As such, treaties and customs consolidate their positions as indispensable sources of international law and justify the position of importance accorded to them in the system of international justice.


[1]Malcolm Shaw,International Law,BRITANNICA,(23 June, 2019, 5:15 PM),

[2]Statute Of The International Court of Justice, International Court of Justice, ( 23 June, 2019, 4:34 PM),

[3]Vienna Convention Treaties, OAS, (23 June, 2019, 3:40 PM), conventionTreaties.html.

[4]Marlon Konchellah, Customs and Treaties Of International Law, Legal Mind, (23 June, 2019, 2:20 PM),

[5]The Sources of International Law, Law Teacher, (23 June, 2019, 10:00 AM),

[6]Sources of International Law,Diakonia, (23 June,2019, 1:00 PM),

[7]Vienna Convention Treaties, OAS, (23 June, 2019, 3:40 PM), conventionTreaties.html.

[8]North Sea Continental Shelf Cases, International Court of Justice, (21 June, 2019, 3:44 PM),

[9] Id.


Submitted by Anomitra Debnath

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