# Public International Law/Sources of International Law/Customary Law

Author: Victor Stoica
   Required knowledge: Public International Law/History of International Law ; Public International Law/Nature and Purpose of International Law; Consent; Enforcement
   Learning objectives: After studying this Chapter, students will be able to understand what customary international law as a source of international law is, and who directly and indirectly contributes towards its formation and identification.

## A. Introduction

Customary international law is unwritten; it is tacit agreement. Prior to World War II, customary international law represented the main mechanism through which international law was created and, during those days, the repeated behaviors of states (or practice) were sufficiently capable of creating legal norms, which, once emerged, contained rights and obligations. In this context, certain opinions exist in the sense that 'a careful examination of history suggests that to a large degree publicists and powerful nations ignored inconvenient state practice and generated customary international law norms based on prior assumed values or perceived self-interests irrespective of the general acceptance of a norm'. It could, therefore, be argued that what the current framework of customary international law is, to a certain degree, the result of a rather regionalized state practice which, in time and in part, became general. In this regard, from a historical perspective, modern customary international law emerged in the 19th century in Europe as a direct result of European philosophy and the traditions of civil Roman law. This practice 'became general' by colonial domination and by the way in which European states resisted the efforts of newly independent states in the 1950s and 1960s to participate in the custom-making and codification process on their own terms.
Today, customary international law is no longer the primary source of international law, but remains one of the most important sources, along with treaties, its importance renewed in recent times of classic treaty-making. In this regard, customary international law offers binding rules of public international law irrespective of hyperpolitical treaty negotiations. Article 38 (1) of the Statute of the International Court of Justice ('the ICJ'), is essential for understanding the meaning and content of customary international law. This provision is not necessarily clear, and has been subject to the interpretation of the ICJ and of the International Law Commission of the United Nations ('the ILC'). Even so, what is currently clear is that the binding force of international customary law rests on the implied consent of states.
The generally accepted interpretation of customary international law confirms that a two-pronged test must be performed in order to determine the existence and content of a rule. First, practice among states must exist and, second, this practice must be accepted as law. In other words, for customary international law to exist, practice must be performed out of a sense of an existing legal obligation. In this sense, a norm of customary international law may bloom if a sufficient number of states perform actions or knowingly abstain from performing an act. States must perform these actions or abstentions in a generally consistent manner, which is sufficiently widespread and representative. They must also do so with the sense that they have an international legal obligation to act in such a way. Framing customary international law in these terms has been labeled as the orthodox perspective on the formation of customary international law. This view is not without controversy, especially because the interaction between practice (referred to as the objective or material element) and its acceptance as law (referred to as the subjective element or opinio iuris) is not easily distilled.
The question regarding who creates customary international law is also rather controversial. The traditional limitations regarding the exclusion of other entities from the formation of customary international law (for example, international organizations or international courts and tribunals) may place customary international law, as a source, on its fringes. For this reason, the legitimacy of customary international law is sometimes considered to be under threat. The practice of international courts and tribunals, including the ICJ, or the manner in which international organizations act, do not necessarily provide profound clarifications regarding a coherent interpretation of this source of international law.
One of the main reasons for this rather seemingly convoluted understanding of customary international law is that, as opposed to treaties, the formation of customary international law does not follow a predictable path, or an exact and regulated procedure. It may seem that customary international law blooms slowly and appears abruptly, at a particular point in time, while it is created behind the curtains of international relations, with no precise indication regarding its appearance. While the traditional view is that the creation of custom essentially entails a substantial amount of time to pass for its creation, recent doctrine has also acknowledge the possibility for the creation of an 'instant custom', in certain emerging domains such as space law. The United Nations General Assembly, through is Resolutions, is also regarded as a main contributor to the creation of instant customs.
International scholarship, addressing the understanding of customary international law, is yet to reach common ground. A notable mentioned must be given to the efforts on the codification of customary international law. The codification of international law, in general, involves identifying and systematising the rules of public international law. This represents an effort on the part of the relevant institutions to provide a greater degree of transparency and certainty to the international law system. International institutions involved in the codification and progressive development of international law, such as the ILC or the International Law Association ('the ILA') have included the identification of customary international law and, respectively, the Formation of Customary (General) International Law, on their agenda. The ILC issued, in 2018, the Draft conclusions on Identification of Customary International Law ('the Draft Conclusions') and the ILA has issued, in 2000, the Statement of Principles Applicable to the Formation of General Customary International Law.
The strict identification of practice and the distinctions between actions and declarations of states contribute to the difficulties in identifying the subjective element, which can be assimilated to beliefs of states, is noticeable when looking towards the direction of the ICJ or the ILC. It has been argued, in this sense, that 'it would be a mistake to believe that power is the only relevant factor in the emergence of CIL. The role of ideas and beliefs is significant'. Further, the ICJ also referred to the beliefs and feelings of states, which crystallize opinio juris as a constituent element of customary international law, throughout its jurisprudence, in cases such as the North Sea Continental Shelf Cases or in the Asylum Case, respectively. Illustratively, in the North Sea Continental Shelf, the Court concluded that belief is needed for the existence of opinio iuris to be demonstrated. Even so, the ICJ has failed to clearly indicate what constitutes opinio iuris and how it interacts with the practice of states in order to determine the content of customary international law. This contributes to the lack of clarity and consensus that revolves around the formation and identification of customary international law. It is still not clear, as 'different meaning and weights are often assigned to the two elements of states practice and opinio iuris'.
This chapter does not endeavor to suggest a new conceptual framework regarding the identification and formation of customary international law. Nor does it intend to resolve the apparent controversies regarding the subject. It rather intends to describe the contemporary manner in which customary international law is formed and identified, by referring, mainly to what is generally accepted.

## B. Constitutive Elements of Customary Law

For customary international law to be formed, two elements must exist: I. A general practice, usually regarded as the objective element and II. Opinio iuris, known as the subjective element. The following sections shall address the content of the two elements.

### I. General Practice

Article 38 of the Statute of the ICJ prescribes that, when deciding the disputes that are submitted to it, the ICJ will apply 'international custom, as evidence of a general practice accepted as law.' The first issue that should be determined when addressing the existence of customary international law is, thus, the indispensable element of general practice.
General practice is considered as the material (or objective) element of customary international law, because its existence does not depend on the intention of states, international organizations or other entities that exist in today's international society. In other words, when practice is performed, it may, usually, be observed. For example, when a state repeatedly transits with its naval ships a certain maritime zone, that is practice.

#### 1. What may count as 'practice'?

It is generally accepted that state practice is primarily relevant for the creation of customary international law, not the practice of international organizations, transnational corporations, not even the practice of international courts and tribunals. Draft Conclusion 4 of ILC's Draft Conclusions confirms this view when concluding that 'the requirement of a general practice, as a constituent element of customary international law, refers primarily to the practice of States that contributes to the formation, or expression, of rules of customary international law.' The ICJ, in the judgment it issued in the Military and Paramilitary Activities in and against Nicaragua concluded in the same terms, that 'to consider what are the rules of customary international law applicable to the present dispute … it has to direct its attention to the practice and opinio iuris of States'.
This finding has led commentators, and certain states, to conclude that only and exclusively states may create practice relevant for the formation of customary international law. However, some entities (such as international courts and tribunals) are relevant for the creation of customary international law while others (such as international organizations) are directly involved in the creation process.
States are abstract entities, with no material form through which they could manifest their activities. States behave through various mechanisms, created and disseminated by multiple stakeholders. Generally speaking, the executive branch of a State is regarded as the primary and most relevant actor in the international law scene.High officials, such as Heads of State or Ministers, municipal courts and tribunals or legislative bodies may be viewed as the limbs, through which states act and through which they have the possibility of developing practice. Not all actions performed by states are capable of creating the material element of customary international law. Scholars have concluded, in this sense, that a distinction should be made between actions and statements/declarations. It is often the case that actions involve a predominantly legal perspective, while statements and declarations involve a predominant political perspective, the latter being less capable of creating practice. This should not lead to the conclusion that statements never contribute to practice, or that they are always political, as unilateral acts have the potential to create international obligations.

#### a) Actions or doing

State practice is generally understood by looking at the acts or actions performed by the executive branch and other executive and legislative bodies. It is also generally accepted that certain high officials have the vocation to represent the state in the international arena. Usually, Heads of States, Prime Ministers or Ministers of Foreign Affairs (usually referred to as 'the Big Three') ( have the power, without the need of full powers, to act on behalf of the state. But not only the actions performed by these official may represent practice. If a national court maintains its jurisprudence regarding a particular aspect, that has international implications, that body of judgments may contribute to the crystallization of state practice on a certain topic.
The ILC confirms, in Draft Conclusion 5, that practice may manifest in situations in which states exercise multiple activities, including through their 'executive, legislative, judicial or other functions.' This list, provided by the ILC, is open ended and implies that any act or action of states, through their organs, may constitute state practice. This is to say that practice is usually analyzed holistically, on a case by case basis, by looking in the direction of the state as a whole, with all its relevant institutions and officials. Illustratively, diplomatic correspondence on a certain issue, along with political declarations of a high official or the positions maintained throughout the negotiation of a treaty on the same subject matter, may also constitute practice.
It is worth mentioning, at this juncture, that states and their organs are abstract concepts which exercise their functions through people, mandated in this respect by the state (for example, through the Constitution), which is, in turn, structured and mandated by its citizens, through the voting process. Consequently, it is the individual/s which represent the organs of states or states themselves, which is/are capable of creating customary international law. Who qualifies as an individual which may represent the State in its external affairs depends on the national legislation of each State. What is essential is that these individuals, who may represent the state and, consequently, who may generate practice, act in their official capacity.
Several acts or actions may illustrate the formation of practice as an element of customary international law. The classic example regarding the formation of customary international law is maritime law, which, when it first emerged, was 'almost entirely customary international law'. Other examples include 'actively seizing foreign vessels, actually expropriating foreign property or sending satellites into orbit', as elements of practice. The North Sea Continental Shelf Cases, in which the exercise of certain fishing rights in economic zones of the coastal states was addressed by the Court, is relevant from this perspective. Not only physical acts (exercising fishing by states or seizing foreign vessels) may constitute practice as an element of customary international law, but legal acts may have serve the same purpose. Illustratively, if states enact legislation through which fish are protected within 200 miles off their coasts, there is potential for the creation of a rule of customary international law in this respect.
However, the distinction between the actions of the State as forming state practice and the actions that prove opinio juris are not always clear and, as a result, there is no universally accepted and applicable theory to rely upon when determining which actions should be regarded as state practice and which should be considered proof of opinio juris.
To summarize, action as an element of customary international law encompasses a multitude of possibilities. This open-ended character should not be regarded as negative, especially since the manner in which states act has diversified and continues to diversify.

#### b) Inactions or not doing

Sometimes, omissions may also represent state practice. In other words, it may happen that silence produces legal effects and, among these, the creation of legal norms. Draft Conclusion 6 issued by the International Law Commission regarding the identification of customary international law confirms that practice may, 'under certain circumstances, include inaction'. Not every inaction constitutes practice for the purposes of creating customary international law and certain distinctions should be made in this sense. The question that would need an answer is therefore: "which omissions may constitute practice"?
It is first relevant to point out that the ICJ has dealt with this question throughout its jurisprudence. One of the more relevant findings in this respect is to be found in the Temple of Preah Vihear Case, in which the Court concluded in the following terms:
   It has been contended on behalf of Thailand that this communication of the maps by the French authorities [note: the sovereignty of Cambodia was under French protectorate at the relevant time] was, so to speak, ex parte, and that no formal acknowledgment of it was either requested of, or given by, Thailand. In fact, as will be seen presently, an acknowledgment by conduct was undoubtedly made in a very definite way; but even if it were otherwise, it is clear that the circumstances were such as called for some reaction, within a reasonable period, on the part of the Siamese authorities, if they wished to disagree with the map or had any serious question to raise in regard to it. They did not do so, either then or for many years, and thereby must be held to have acquiesced. Qui tacet consentire videtur si loqui debuisset ac potuisset.
The ICJ confirmed its finding from the Temple of Preah Vihear Case in other judgments, such as the one in the Case Concerning Sovereignty over Pedra Branca/Pulau Batu Puteh, Middle Rocks and South Ledge (Malaysia/Singapore), in which it held that 'silence may also speak, but only if the conduct of the other State calls for a response.' The Commentary of the Draft Conclusions further provides examples in this sense: 'abstaining from instituting criminal proceedings against foreign State officials; refraining from exercising protection in favor of certain naturalized persons; and abstaining from the use of force', may lead to the creation of customary international law through inaction. The answer that seems to be confirmed by the doctrine is that only the omissions which are clear in their scope may fulfil the role of practice as an element of customary international law. In this sense, omissions must be carefully interpreted so as to determine what was the true intention of the State that did not perform a particular action. The caution needed for addressing omissions as practice used for the creation of customary international law is demonstrated by the practice of international courts and tribunals. In this respect, the Permanent Court of International Justice, in the Lotus Case, did not address the formation of customary international law because the abstention in question was ambiguous. The International Court of Justice was braver in its analysis of inaction as evidence of practice. In the Temple of Preah Vihear Case, the Court concluded that inaction may generate rights and obligations only if that state must and can act, but it does not. The Court decided that 'it is clear that the circumstances were such as called for some reaction, within a reasonable period, on the part of the Siamese authorities, if they wished to disagree with the map or had any serious question to raise in regard to it. They did not do so, either then or for many years, and thereby must be held to have acquiesced.'
To summarize, as opposed to actions, all of which may constitute practice for the purposes of creating customary international law, not all inactions may contribute to this process. Only clear inactions, which call for a response, may represent practice.

#### c) Statements

As initially concluded by Anthony D'Amato and further confirmed by Anthea Roberts statements are rather included under the subjective element of international law, opinio iuris. Draft Conclusion 10 issues by the ILC confirms that 'public statements made on behalf of States' represent one of the multiple forms of evidence as acceptance as law. As such, the relevance of public statements for the creation of customary international law will be treated below, under the section entitled 'Accepted as Law (opinio iuris)'.
Different opinions also exist. Illustratively, Mendelson concludes that statements may constitute either state practice or opinio iuris and that 'whether we classify a particular verbal act as an instance of the subjective or of the objective element may depend on circumstances'. For example, a statement of a Head of State of that of a Foreign Minister, should constitute practice rather than opinio iuris while the one of a junior diplomat should not. What is important, he argues, is that the same statement is not treated as both practice and opinio iuris, in order to avoid double counting and to place an unjustified weight on such statements.
As such, the traditional perspective regarding statements is that they may not constitute state practice and that they should rather be treated under the expression on opinio iuris, or the belief of states that a conduct is mandatory under international law. However, a more progressive view, which is justified, is that statements (especially those of high officials) may constitute state practice. In support of this, certain scholars point out that there are 'important acts of state behaviour, such as recognition of another state, do not need a physical act'.

#### 2. What does 'general' mean?

Part of the international scholarship accepts that generality of practice means that 'all or almost all of the nations of the world engage in it'. Even if this conclusion has certain merit, the same authors conclude that it is impossible to determine if more than 190 states have engaged in a certain practice, which, as seen above, is complex and can be manifested through a wide range of activities or abstentions. Further, practice is rarely virtually homogenous.
This is the reason for which Draft Conclusion 8 provides that for practice to be general is must be 'sufficiently widespread and representative as well as consistent'. This three pronged standard provided by the ILC is, thus, lower than the one related to the unanimity or majority of practice. As such, generality implies three different tests that should be met cumulatively. The Commentary of the Draft Conclusions does not further qualify what these three concepts mean but mentions that 'no absolute standard can be given for either requirement', argument that leads to the conclusion that their content is addressed on a case by case basis, leaving international courts and tribunals with a wide degree of appreciation for the consideration of the existence of a rule of customary international law.

#### a) Sufficiently widespread

Widespread practice is generally understood as 'existing or happening in many places and/or among many people'. The ICJ confirmed the relevance of the concept in the Case Concerning Maritime Delimitation and Territorial Questions between Qatar and Bahrain, in which it concluded that it was not aware of the existence of uniform and widespread practice that could lead it to conclude that a customary rule regarding the appropriation of low-tide elevations exists. In Questions relating to the Obligation to Prosecute or Extradite the ICJ concluded, with respect to the imperative norm regarding the prohibition of the use of torture, that it is 'grounded in a widespread international practice and on the opinio iuris of States'.
Even so, the ICJ has not endeavored to define the concept, nor does the Commentary of the Draft Conclusions. It just quotes the North Sea Continental Shelf cases, in which the ICJ concluded that the practice in question must be 'both extensive and virtually uniform', or 'settled practice'. These standards are not universal, nor were they relevant in all cases in which the application of customary international law was at stake. The Commentary goes even further and mentions that 'no absolute standard can be given for either requirement' . The only clarification provided is that practice is sufficiently widespread when it is not 'contradictory or inconsistent'.
It would seem that the first standard is defined through what it is not. In other words, practice may be widespread as long as it is not limited. This manner of determining the content of a concept is usually designed by international courts and tribunals in order to allow a certain degree of appreciation.

#### b) Sufficiently representative

Representative practice is generally understood as 'typical of, or the same as, others in a larger group of people or things'. At first glance, it might seem that the concept of 'representative' has common features with the concept of 'widespread', especially because the number of entities participating in the creation of customary international law is relevant in both cases. However, certain distinctions should be drawn between the two. Representative practice is rather qualitative in nature, as opposed to widespread practice, which is rather quantitative in nature.
The ICJ has not necessarily addressed what 'representative' means. In North Sea Continental Shelf, the ICJ concluded that 'a very widespread and representative participation' would be sufficient to demonstrate the existence of customary international law resulting from a convention. The Commentary of the Draft Conclusions provides that for the practice to be representative it must take into consideration the 'various interests at stake and/or the various geographical regions.' This approach confirms the quantitative/qualitative distinction mentioned above. Therefore, for practice to be representative, the approach of certain states has more weight while the approach of other less so.

#### c) Consistency

Consistency is generally understood as 'the quality of always behaving or performing in a similar way, or of always happening in a similar way'. This standard implies that practice should manifest stability over time. As such, if the behaviors of states fluctuate over time, it would be difficult, if not impossible, to identify a general practice. The question, here, is whether there is a need for uniformity of practice (complete consistency) for the formation of customary international law or whether a lower standard should be applied.
The ICJ has not provided any clarification regarding the content of this concept, but rather discussed the level of consistency necessary for the formation of customary international law. In Military and Paramilitary Activities the ICJ concluded in the following terms:
   It is not to be expected that in the practice of States the application of the rules in question should have been perfect in the sense that States should have refrained, with complete consistency [...]. The Court does not consider that, for a rule to be established as customary, the corresponding practice must be in absolutely rigorous conformity with the rule. In order to deduce the existence of customary rules, the Court deems it sufficient that the conduct of States should, in general, be consistent with such rules
This finding was confirmed in the North Sea Continental Shelf Cases, in which the court did not refer to 'consistency' but to 'uniformity', as a similar concept. The Court concluded that:
   Although the passage of only a short period of time is not necessarily, or of itself, a bar to the formation of a new rule of customary international law on the basis of what bras originally a purely conventional rule, an indispensable requirement would be that within the period in question, short though it might be, State practice, including that of States whose interests are specially affected, should have been both extensive and virtually uniform in the sense of the provision invoked.
So, consistency means virtual uniformity, in accordance with the practice of the ICJ, in the sense that there is no need for global practice for the formation of customary international law.

### II. Accepted as Law (Opinio Iuris)

For customary international law to exist, first, general practice must be present, as provided above, under Section I. But practice should also be performed out of a sense of a legal obligation. This perception, that states have a legal obligation, is qualified as the subjective element of customary international law, or opinio iuris, which has been labeled as belief.
The Commentary of the ILC mentions that customary international law cannot exist in the absence of opinio juris, by arguing that 'the presence of only one constituent element does not suffice for the identification of a rule of customary international law.'
The ICJ referred to the perceptions that states have, in order to determine how opinio iuris forms, as a constituent element of customary international law, throughout its jurisprudence, in cases such as the North Sea Continental Shelf Cases or in the Asylum Case, and it confirmed that opinio iuris must exist in order to determine that customary international law exists. Illustratively, the Court concluded that:
   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 requiring it. The need for such a belief, i.e., the existence of a subjective element, is implicit in the very notion of opinio iuris sive necessitatis. The States concerned must therefore feel that they are conforming to what amounts to a legal obligation.
The ILC refers to the above mentioned conclusion, but does not shed much light into what exactly may serve as a tool in order to determine what are the perceptions, beliefs or feelings of states. As such, Draft Conclusion 10 firstly provides that opinion iuris 'may take a wide range of forms' and that 'public statements made on behalf of states; official publications; diplomatic correspondence; decisions of national courts; treaty provisions; and conduct in connection with resolutions adopted by an international organisation or intergovernmental conference' can be used as elements through which opinio iuris may be identified. The ILC also provides that lack of action in circumstances in which states "were in a position to react" may also indicate the presence of opinio iuris.
How does one determine what a state believes, or considers, to be mandatory and what not? Scholars conclude that this element of custom is controversial, and that 'no one dares question that its verification is necessary for a customary rule to exist'. The distinction between acts (such as the ones enumerated above) that confirm the perception of states of binding international legal obligations and the perception of states according to which they are acting out of courtesy is also not clear. In other words, the list of acts that may crystalize opinio iuris is uncertain, to the degree that the same act may or may not lead to its formation. The International Court of Justice, in the North Sea Continental Shelf Cases, confirmed that 'there are many international acts, e.g. in the field of ceremonial and protocol, which are performed almost invariably, but which are motivated only by considerations of courtesy, convenience or tradition, and not by any sense of legal duty". As such, "it is difficult to determine what states believe as opposed to what they say.'
These uncertainties regarding a precise manner in which opinio iuris should be determined have led certain authors to conclude that the subjective element should be less relevant, and that practice should be at the forefront of identifying customary international law. Nevertheless, it is rather generally accepted that 'while opinio juris confers the legal bindingness of custom, practice, it is argued, can be understood as what provides custom with normative content.' In other words, while practice provides what the norm contains, opinio iuris is what confers to that norm its binding character. In this context, opinio iuris is essential for the creation of customary international law.

## C. Who is bound by custom?

The clarification on the issue of the creation of international custom isn't the only aspect of international customary law which entails complexity. In particular, the problem of the binding character of such rules in relation to certain category of States is also one which needs further analysing, in particular, the issue of the silence of a State in relation to a custom (also known as aquiesence), the persistent objector and the special case of States particularly affected by a certain rule of customary international law, issues which will be discussed below.

### I. Aquiesence

Silence in relation to the formation of customary international law is often labelled as acquiescence. In addition to all the elements of practice, including the one related to the capacity of the individual which partakes in its formation, representing the state, commentators have concluded that acquiescence exists in situations in which several conditions are met: if the facts were known or should have been known, if these facts were in the interest of the state and if these facts were consistent. Other standards have been described by commentators concluding that inaction may generate customary international law if there exist 'intention and deliberateness'.

### II. The persistent objector

Inconsistent practice sometimes manifests itself through the persistent objector doctrine. Conclusion 15 of the ILC Draft Conclusions on Identification of Customary International Law provides that if a State expressly objects to a rule of customary international law when that rule is in the process of formation, the said rule will not be applicable to that state.
The distinction is relevant, because potential objections of states which are performed after customary international law was formed are no longer relevant. In other words, states that did not object during the formation of customary law do no longer have the right to object when they must comply with the created rules. The role of the consistent objector doctrine is that it respects States’ sovereignty and protects them from having new law imposed on them against their will by a majority; but at the same time, if the support for the new rule is sufficiently widespread, the convoy of the law’s progressive development can move forward without having to wait for the slowest vessel.'

### III. The case of specially affected States

The Commentary of the ILC Draft Conclusions sheds light into the relevance of this concept, in the following terms:
   In assessing generality, an important factor to be taken into account is the extent to which those States that are particularly involved in the relevant activity or most likely to be concerned with the alleged rule have participated in the practice. It would clearly be impractical to determine, for example, the existence and content of a rule of customary international law relating to navigation in maritime zones without taking into account the practice of coastal states and major shipping states.
For example, the rise of the level of seas and oceans imply significant threads to small island developing States for multiple reasons, including geographical, related to the concentration of people and infrastructure present in coastal areas. These States may be considered as specially affected for the creation and identification of customary international law related to sea level rise. The current work of the International Law Commission on Sea Level Rise in Relation to International Law confirms that the Pacific Island States are specially affected and that their practice is relevant. This is not to argue that the specially affected states are the only ones that contribute to the creation of international law in a particular field but that their practice should carry more weight than the practice of states that do not have this status. In other words, states that are affected indirectly, states that are not at the forefront of danger, should observe the practice of specially affected states.

## D. Who creates custom?

Article 38 (1) b. of the Statute of the ICJ prescribes that the Court will decide in accordance with international law by applying „international custom, as evidence of a general practice accepted as law”. The mentioned provision does not explicitly mention that states, or international governmental organizations, or both, are the only entities that can participate in or influence the formation of customary international law. Nevertheless, what is traditionally accepted is that states and international governmental organizations are the main subjects of international law. Certain authors confirm that the two entities are the only subjects of international law, strictly speaking and, as such, the only ones that can contribute to the formation of customary international law.

### I. States

The practice of the ICJ confirms that states are the entities that contribute to the formation of customary international law. In the Asylum Case, the ICJ found that customary international law did not exits because the practice of states was not uniform and constant. In the North Sea Continental Shelf, the ICJ concluded that 'State practice, including that of States whose interests are specially affected' is relevant for the formation of customary international law. Further, in Military Activities in and Against Nicaragua, the Court referred to the practice of States and to opinio iuris of States while in Jurisdictional Immunities it also determined that 'State practice of particular significance is to be found in the judgments of national courts faced with the question whether a foreign State is immune, the legislation of those States which have enacted statutes dealing with immunity, the claims to immunity advanced by States before foreign courts and the statements made by States'.
Thus, the jurisprudence of the International Court of Justice confirms that states are involved in the formation of customary international law. The former President of the Court, Judge Yusuf concluded in his Statement before the Sixth Committee that customary international law, at least in accordance with the 'old approach' was primarily based on the consent of states, while other entities, such as the United Nations provide states with a forum through which they engage on different issues, including the formation of customary international law.
The ILC, within its Draft Conclusions regarding the identification of customary international law confirms that states contribute to its formation. In this respect, Draft Conclusion 4 paragraph 1 provides that practice primarily refers to States while the Commentary of the Draft Conclusion further clarifies that States have a pre-eminent function for the formation of customary international law. The wording does not exclude other enteritis that might contribute to the formation of international law. Thus, international organizations may also influence the formation process.

### II. International Organizations

Not only States may contribute to the formation of customary international law. As secondary subjects of international law, international organizations may also influence the manner in which customary international law is generated. Out of all the subjects of international law, the international organization is the only one that manifests a double role: direct and indirect. As such, first, international organizations may influence how state contribute to the formation of international law (indirect influence) or, in situations in which international organizations behave like states they directly contribute to the formation of customary international law.

### 1. Indirect influence

International organization may influence the formation of customary international law by impacting the conduct of their member states. As such, international organizations may be considered as 'catalysts of state practice'. In other words, it has been argued that international organizations shape what states do and think. Illustratively, the European Union regularly participates in international legal forums, such as the Sixth Committee of the United Nations or the International Law Commission, bodies of the UN which address and discuss the norms of international law.
The ILC Articles on State Responsibility confirms this possibility through the Commentary of the Draft Conclusions which provides that 'resolutions adopted by international organization or at intergovernmental conferences, even when devoid of legal force of their own, may sometimes play an important role in the development of customary international law'. The issue of indirect influence is not necessarily controversial and is recognized by the international community. This is not the case with direct influence.

### 2. Direct influence

The International Law Commission, through Draft Conclusion 4 para. 2 confirms that international organization may directly influence the formation of customary international law. As such, the mentioned Conclusion provides that international organisations, through their practice, contribute to the expression of customary international law. Further, the Commentary of the Draft Conclusions clearly delineates direct influence from indirect influence, as it mentions that the above mentioned paragraph concerns 'practice that is attributed to international organizations themselves, not practice of States acting within or in relation to them'. The European Union is often considered as being such an organization which, in certain circumstances, or for certain competences, acts in the international arena like a State.
The direct influence of international organizations regarding the formation of customary international law is not as clear cut as the indirect influence. Illustratively, the United States of America have criticized the approach of the International Law Commission and mentioned that 'it is axiomatic that customary international law results from the general and consistent practice of States followed by them out of a sense of legal obligation'.
Even so, the doctrine rather supports the view that international organizations have the possibility to directly influence the formation of customary international law. Further, the majority of states endorsed the approach of the ILC and 'affirmed that at least some international organizations can sometimes contribute directly to the formation of customary international law'.

### III. International Courts and Tribunals

International Courts and Tribunals do not contribute, directly, to the formation of customary international law. Draft Conclusion 13 of the ILC confirms that 'Decisions of international courts and tribunals, in particular of the International Court of Justice, concerning the existence and content of rules of customary international law are a subsidiary means for the determination of such rules'. This means that the jurisprudence of international courts and tribunals are not part of the objective element (practice) or of the subjective element (opinio iuris).
Nevertheless, they have a role in the identification of the rules of customary international law. In other words, it may be said that international court and tribunals may serve as a magnifying glass through which custom is seen. But, this should not lead to the conclusion that 'his is customary international law because the ICJ said so'. Consent is one of the fundamental pillars of the formation of customary international law. In this context, if the International Court of Justice concludes that there exists a norm of customary international law and states (including the parties to the dispute) accept this norm, either implicitly or explicitly, it may evolve into customary international law.

## E. Special Customary International Law
### I. Jus Cogens Norms

One of the limits is represented by jus cogens norms which are globally widespread, and are part of the concept of general customary international law. Jus cogens norms or the peremptory norms of public international law are rules 'accepted and recognized by the international community as a whole (...) from which no derogation is permitted'. The reside at the top of the hierarchy of international law norms. These customary norms 'are considered so vital that they cannot be contracted out of by individual states'; they have the highest degree of generality.

### II. Regional Customary International Law

At the opposing end of the spectrum rests regional (or particular) customary international law. In this regard, the ICJ recognized the possibility of a custom to exist on a regional or even a bilateral basis, practice which may be based upon a need for 'respect for regional legal traditions'. In 1950, in the Judgment issued in the Asylum Case the Court did not exclude the general possibility of regional customs to exist, even if it concluded that the Colombian Government did not prove the existence of such a custom. In 1960, in the Right of Passage over the Indian Territory case, the ICJ held that 'where therefore the Court finds a practice clearly established between two States which was accepted by the Parties as governing the relations between them, the Court must attribute decisive effect to that practice for the purpose of determining their specific rights and obligations'. In 1984, in the judgment issued in the Case concerning Military and Paramilitary Activities in and against Nicaragua, the Court appeared to agree that regional customary international law, 'particular to the inter-American legal system', exists. Conclusion 16 of the Draft Conclusions on Identification of Customary International Law qualified regional customary international law as part of the particular customary international law that applies only to certain States.

## F. Conclusion

There are no clear, universally applicable, strictly rigorous and fixed rules for the creation of customary international law. However, the lack of such parameters is not in itself a disadvantage given the ever-evolving nature of public international law. Customary international law is characterised by agility, and has the potential to address multiple legal frameworks: it may be regional or global, it may be confirmed through treaties or detached from them, it may be general or special. Consequently, flexibility in the identification of customary law may appear suitable, as it reflects the ever changing developments of international law and policy. This is the reason for which historical approaches are useful for addressing the content of customary international law. As the ICJ held, 'It is, thus, not important for the Court to develop a coherent methodology of identifying customary international law. Instead, identification strategies may differ depending on the circumstances of each individual case and the preferences of the affected parties'.
Moreover, the role of the Court in identifying customary law is essential: 'Customary law, being vague and containing gaps compared with written law, requires precision and completion about its content. This task, in its nature being interpretative, would be incumbent upon the Court. The method of logical and teleological interpretation can be applied in the case of customary law as in the case of written law'. Even if customary international law is created by states and international governmental organisations and influenced by other entities acting on the stage of international law, this source is fragile and should be carefully addressed by international courts and tribunals.
Summary
* Customary international law may form and may be identified if two elements are observed: practice and opinio iuris. Practice is generally understood as practice of states, but the practice of international organisations is also relevant for the creation of customary international law. Further, even if to the same degree, and not as directly, other entities that play in the field of international relations, lato sensu, such as individuals or non-governmental organisations, contribute to the creation and interpretation of customary international law through their actions and inactions.
* Practice may take a variety of forms. Currently, there is no clear and exact determination of what counts as practice. However, actions or inactions performed or omitted by states through their legislative or executive bodies are generally accepted as generators of practice. The Big Three (President, Prime-minister, Minister of Foreign Affairs) represent states internationally and consequently, their actions/inactions are especially relevant.
* Practice must be general, in order to contribute to the formation of customary international law. Generality implies that practice is sufficiently widespread, sufficiently representative and consistent. However, this should not lead to the conclusion that only global customary international law exists but that these elements should also be regionally calibrated and, further, that specially affected states (or states that have a direct and serious interest) should be deferred to in certain circumstances.
* Opinio iuris is represented by the perception of states regarding the nature of a certain practice; if states perform practice out of a sense of an existing international legal obligation customary international law may bloom.
* Customary international law is agile, flexible and sometimes vague. This may lead to the conclusion that custom is not a 'serious' source of international law. However, this argument has several fallacies. Customary international law was, is and will be at the core of international law. Several existing treaties, such as the Vienna Convention on the Law of Treaties, the United Nations Convention of the Law of the Sea or the Vienna Convention on Diplomatic Relations, confirm that customary international law is the headspring of what may become, in time, and with the fulfilment of certain conditions, a river.

## Further Readings

* James Crawford, Chance, Order, Change: The Course of International Law, General Course on Public International Law, Brill, 2014
* Jean D'Aspremont, International Law as a Belief System, Cambridge University Press, 2017
* Anthea Roberts, Is International Law International, Oxford University Press, 2017
* Hersch Lauterpacht, The Function of Law in the International Community, Oxford Public International Law, 2011

## Table of Contents

Back to home page
Part I - History, Theory, and Methods
* § 1 History of International Law
** § 1.1 Founding Myths
** § 1.2 Nineteenth Century
** § 1.3 Decolonization
* § 2 Nature and Purpose of International Law
** § 2.1 International Law and Violence
** § 2.2 Consent
** § 2.3 Enforcement
* § 3 Methodology
** § 3.1 Case Analysis
** § 3.2 Interdisciplinarity
* § 4 Approaches to International Law
** § 4.1 Positivism
** § 4.2 TWAIL
** § 4.3 Feminism & Queer Theory
** § 4.4 Marxism
* § 5 Interaction
** § 5.1 Interaction within International Law
** § 5.2 International Law and Domestic Law
Part II - General International Law
* § 6 Sources of International Law
** § 6.1 Treaty Law
** § 6.2 Customary Law
** § 6.3 General Principles
** § 6.4 Other Sources
* § 7 Subjects and Actors in International Law
** § 7.1 States
** § 7.2 Indigenous Peoples
** § 7.3 International Organizations
** § 7.4 Individuals
** § 7.5 Women in International Law
** § 7.6 Non-governmental Organization
** § 7.7 Corporations
** § 7.8 Animals
** § 7.9 Cities
* § 8 Jurisdiction
* § 9 State Responsibility
* § 10 Diplomatic Relations
* § 11 Immunities
* § 12 Peaceful Settlement of Disputes
** § 12.1 Evidence
* § 13 Use of Force
Part III - Specialized Fields
* § 14 Law of Armed Conflict
* § 15 Law of the Sea
* § 16 International Environmental Law
* § 17 International Climate Change Law
* § 18 International Migration Law
* § 19 International Law in Cyberspace
* § 20 Space Law
* § 21 Human Rights Law
** § 21.1 Recurring Themes in Human Rights Doctrine
** § 21.2 Institutional Human Rights Protection
*** § 21.2.1 United Nations Human Rights System
*** § 21.2.2 African Human Rights System
*** § 21.2.3 European Human Rights System
*** § 21.2.4 Inter-American Human Rights System
*** § 21.2.5 Arab and Islamic Human Rights System
*** § 21.2.6 Asian Human Rights System
** § 21.3 Critique of Human Rights
* § 22 International Criminal Law
** § 22.1 International Crimes
** § 22.2 International Criminal Courts and Tribunals
** § 22.3 Domestic Prosecution of International Crimes
* § 23 International Economic Law
** § 23.1 Investment Law
** § 23.2 Trade Law
** § 23.3 Monetary Law

## Footnotes

* ↑ J. Pattrick Kelly, Customary International Law in Historical Context, in Brian D. Lepard (ed.), Reexamining Customary International Law, Cambridge University Press, 2017, 47, available at: https://www.cambridge.org/core/books/abs/reexamining-customary-international-law/customary-international-law-in-historical-context/02A51C93813D4889EC5BC79120B3E396
* ↑ Statute of the International Court of Justice, available at: https://www.icj-cij.org/en/statute
* ↑ Vincy Fon, Francesco Parisi, Stability and Change in International Customary Law, University of Chicago, available at:https://www.journals.uchicago.edu/doi/pdfplus/10.1086/656058
* ↑ Jutta Brunnée, Customary International Law Symposium: Making Sense of Law as Practice (Or: Why Custom Doesn’t Crystallize), available at: http://opiniojuris.org/2020/07/07/customary-international-law-symposium-making-sense-of-law-as-practice-or-why-custom-doesnt-crystallize/
* ↑ John Tasioulas, Opinio Iuris and the Genesis of Custom: A Solution to the "Paradox", Australian Yearbook of International Law, 26, 2007, available at: http://classic.austlii.edu.au/au/journals/AUYrBkIntLaw/2007/7.html
* ↑ Draft conclusions on identification of customary international law, with commentaries, International Law Commission, available at: https://legal.un.org/ilc/texts/instruments/english/commentaries/1_13_2018.pdf
* ↑ International Law Association, Statement of Principles Applicable to the Formation of General Customary International Law, available at: https://www.law.umich.edu/facultyhome/drwcasebook/Documents/Documents/ILA%20Report%20on%20Formation%20of%20Customary%20International%20Law.pdf
* ↑ Chimni, B. S. "Customary International Law: A Third World Perspective". American Journal of International Law. 112 (1): 28.
* ↑ North Sea Continental Shelf (Germany v. The Netherlands), Merits, Judgment of 20 February 1969, I.C.J. Reports 1969, p. 3, at p. 44, para. 77, available at: https://www.icj-cij.org/public/files/case-related/52/052-19690220-JUD-01-00-EN.pdf
* ↑ Chimni, B. S. "Customary International Law: A Third World Perspective". American Journal of International Law. 112 (1): 2.
* ↑ Harmen van der Wilt, State Practice as Element of Customary International Law, International Criminal Law Review, 2019, available at: https://brill.com/view/journals/icla/20/5/article-p784_784.xml?language=en
* ↑ Statute of the International Court of Justice, available at: https://www.icj-cij.org/en/statute
* 1 2 3 4 5 6 7 8 9 10 11 12 International Law Commission, Draft Conclusion on Identification of Customary International Law, available at: https://legal.un.org/ilc/texts/instruments/english/commentaries/1_13_2018.pdf
* ↑ Military and Paramilitary Activities in and Against Nicaragua (Nicaragua v. United States of America), Merits, Judgment. I.C.J. Reports 1986, p. 14, para 184, 98, available at: https://www.icj-cij.org/public/files/case-related/70/070-19860627-JUD-01-00-EN.pdf
* 1 2 Shaw, Malcolm N. (2017). International Law (8th ed.). Cambridge University Press. p. 60.
* ↑ Comments from the United States on the International Law Commission's Draft Conclusions on Identification of Customary International Law as Adopted by the Commission in 2016 on First Reading, available at: https://legal.un.org/ilc/sessions/70/pdfs/english/icil_usa.pdf
* ↑ Roberts, Anthea (2001). "Traditional and Modern Approaches to Customary International Law". Americal Journal of International Law. 95 (4).
* ↑ Lucas Lima, Raffaela Kunz, Bernardo Mageste Castelar Campos, Other Sources, available at: Public International Law/Sources of International Law/Other Sources
* ↑ DR Rothwell, T Stephens, The International Law of the Sea (2010) 22; R Barens, D Freestone, DM Ong, ‘The Law of the Sea: Progress and Prospects’ in D Freestone, R Barens, D M Ong (eds), The Law of the Sea. Progress and Prospects (2006)
* ↑ Klabbers, Jan (2020). International Law (3rd ed.). Cambridge University Press. p. 50.
* ↑ International Court of Justice, North Sea Continental Shelf cases (Federal Republic of ­Germany v. Denmark; Federal Republic of Germany v. the Netherlands), Judgment, 20 ­February 1969, 1969 General List Nos. 51 and 52
* ↑ Laurence Boisson de Chazournes, Qu’est-ce que la pratique en droit international?, available athttps://archive-ouverte.unige.ch/unige:36443
* ↑ Ibid.
* ↑ María Vásquez Callo-Müller, What Is the Role of Unilateral Cyber Sanctions in the Context of the Global Cybersecurity Law-Making?, available at: https://voelkerrechtsblog.org/what-is-the-role-of-unilateral-cyber-sanctions-in-the-context-of-the-global-cybersecurity-law-making/
* ↑ Rudolf Bernhardt, Customary International Law, in 1 Encyclopedia of Public International Law, 898, 900
* ↑ Case concerning the Temple of Preah Vihear (Cambodia v. Thailand), Merits, Judgment of 15 June 1962: I.C. J. Reports 1962, p. 6, available at: https://www.icj-cij.org/public/files/case-related/45/045-19620615-JUD-01-00-EN.pdf
* ↑ Sovereignty over Pedra Branca/Pulau Batu Puteh, Middle Rocks and South Ledge (Malaysia v. Singapore), Merits, Judgment of 23 May 2008, I.C.J. Reports 2008, p. 12, para. 121, 50-51 available at: https://www.icj-cij.org/public/files/case-related/130/130-20080523-JUD-01-00-EN.pdf
* ↑ International Law Commission, Draft conclusions on identification of customary international law, with commentaries, 133, available at: https://legal.un.org/ilc/texts/instruments/english/commentaries/1_13_2018.pdf
* ↑ Maurice H. Mendelson, The Formation of Customary International Law, p. 207, available at: https://files.pca-cpa.org/pcadocs/bi-c/2.%20Canada/4.%20Legal%20Authorities/RA-97%20-%20Formation%20of%20Customary%20International%20Law,%20M.%20Mendelson%20(1998).pdf
* ↑ id.
* ↑ Case concerning the Temple of Preah Vihear (Cambodia v. Thailand), Merits, Judgment of 15 June 1962: I.C.J. Reports 1962, p. 6, 23, available at: https://www.icj-cij.org/public/files/case-related/45/045-19620615-JUD-01-00-EN.pdf
* ↑ D’Amato, Anthony A. (1971). The Concept of Custom in International Law. Cornell University Press. p. 49.
* ↑ Roberts, Anthea (2013). Traditional and Modern Approaches to Customary International Law: A Reconciliation.
* ↑ Maurice H. Mendelson, The Formation of Customary International Law, 206, available at: https://files.pca-cpa.org/pcadocs/bi-c/2.%20Canada/4.%20Legal%20Authorities/RA-97%20-%20Formation%20of%20Customary%20International%20Law,%20M.%20Mendelson%20(1998).pdf
* ↑ id.
* ↑ id.
* ↑ Kammerhofer, Jörg (2004). "Uncertainty in the Formal Sources of International Law: Customary International Law and Some of Its Problems" (PDF). European Journal of International Law. 15 (3): 526.
* ↑ Landman Goldsmith, Jack; Posner, Eric A. "A Theory of Customary International Law". University of Chicago Law School, John M. Olin Law & Economics Working Paper (63).
* ↑ Ibid.
* ↑ Niels Peteresen, The International Court of Justice and the Judicial Politics of Identifying Customary International Law, The European Journal of International Law, 2017, vol. 28, no. 2, 377.
* ↑ Draft Conclusions on Identification of Customary International law, available at: https://legal.un.org/ilc/texts/instruments/english/commentaries/1_13_2018.pdf
* 1 2 3 "Cambridge Dictionary".
* ↑ Maritime Delimitation and Territorial Questions between Qatar and Bahrain (Qatar v. Bahrain), Merits, ICJ Reports (2001) 40, at 101–102, para. 205.
* ↑ Questions relating to the Obligation to Prosecute or Extradite (Belgium v. Senegal), Merits, Judgment of 20 July 2012: I.C.J. Reports 2012, p. 422, para. 99, 39, available at: https://www.icj-cij.org/public/files/case-related/144/144-20120720-JUD-01-00-EN.pdf
* ↑ Case concerning the North Sea Continental Shelf (Germany v. the Netherlands), Merits, Judgment of 20 February 1969, I.C.J. Reports 1969, p. 3, para. 74, 44, available at: https://www.icj-cij.org/public/files/case-related/52/052-19690220-JUD-01-00-EN.pdf
* ↑ Ibid. para 77, 45.
* ↑ Ibid.
* ↑ Fon, Vincy; Parisi, Francesco (2009-02). "Stability and Change In International Customary Law". Supreme Court Economic Review. 17 (1): 279–309. {{cite journal}}: Check date values in: |date= (help)
* ↑ ibid.
* ↑ Militarv and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America), Merits, Judgment of 27 June 1986: I.C.J. Reports 1986, p. 14, para. 186, available at: https://www.icj-cij.org/public/files/case-related/70/070-19860627-JUD-01-00-EN.pdf
* ↑ Case concerning the North Sea Continental Shelf (Germany v. the Netherlands), Merits, Judgment of 20 February 1969, I.C.J. Reports 1969, p. 3, para. 74, available at: https://www.icj-cij.org/public/files/case-related/52/052-19690220-JUD-01-00-EN.pdf
* ↑ "ESIL Reflection: Distinguishing the Legal Bindingness and Normative Content of Customary International Law – European Society of International Law | Société européenne de droit international". 2017-12-12. Retrieved 2023-05-30.
* ↑ Case concerning the North Sea Continental Shelf (Germany v. the Netherlands), Merits, Judgment of 20 February 1969, I.C.J. Reports 1969, available at: https://www.icj-cij.org/public/files/case-related/52/052-19690220-JUD-01-00-EN.pdf
* ↑ "Fantastical Opinio Juris and How to Find It". Opinio Juris. 2021-06-23. Retrieved 2023-05-30.
* ↑ Roberts, Anthea. "Traditional and Modern Approaches to Customary International Law: A Reconciliation". {{cite journal}}: Cite journal requires |journal= (help)
* ↑ Mattei-Gentili, Piero (2020-06-15). "The Quest for Opinio Juris: An Analysis of Customary Law, from Hart's Social Rules to Expectations and Everything in the Middle". Noesis (34): 89–114. doi:10.4000/noesis.5154. ISSN 1275-7691.
* ↑ "ESIL Reflection: Distinguishing the Legal Bindingness and Normative Content of Customary International Law – European Society of International Law | Société européenne de droit international". 2017-12-12. Retrieved 2023-05-30.
* ↑ Carter, Barry E.; Weiner, Allen S.; Hollis, Duncan B. (2018). International Law (7 ed.). Aspen Publishing. p. 134.
* ↑ International Law Commission, Draft Conclusion on Identification of Customary International Law, available at: https://legal.un.org/ilc/texts/instruments/english/commentaries/1_13_2018.pdf
* ↑ Rosanne Martyr-Koller, Adelle Thomas, Carl-Friedrich Schleussner, Alexander Nauels, Tabea Lissner, Loss and damage implications of sea-level rise on Small Island Developing States, Current Opinion in Environmental Sustainability, Volume 50, 2021, 245-259, available at: https://www.sciencedirect.com/science/article/pii/S1877343521000713
* ↑ Bogdan Aurescu, Nilüfer Oral, First issues paper on sea-level rise in relation to international law, available at: https://documents-dds-ny.un.org/doc/UNDOC/GEN/N20/053/91/PDF/N2005391.pdf?OpenElement
* 1 2 Colombian-Peruvian asylum case (Colombia v. Peru), Merits, Judgment of 20 November 1950: I.C.J. Reports 1950, p. 277.
* ↑ Case concerning the North Sea Continental Shelf (Federal Republic of Germany v. the Netherlands), Merits, Judgment of 20 February 1969: I.C.J. Rep 1969, p. 43, para. 73.
* ↑ Case concerning Military and Paramilitary Activities In and Against NIcaragua (Nicaragua v. United States of America), Merits, Judgment of 27 June 1986: I.C.J. Rep 1986, p. 108, para. 206.
* ↑ Case concerning Jurisdictional Immunities (Germany v. Italy), Merits, Judgment of 3 February. 2012: I.C.J. Rep 2012, p. 123, para. 55.
* ↑ Statement of H.E. Judge Yusuf before the Sixth Committee of the General Assembly, 1 November 2019, available at: https://www.icj-cij.org/public/files/press-releases/0/000-20191101-STA-01-00-EN.pdf
* ↑ Odermatt, Jed (2021). International Law and the European Union. Cambridge University Press. p. 43.
* ↑ Kristina Daugirdas, International Organisations and the Creation of Customary International Law, EJIL, Vol. 31, no. 1, 202
* ↑ Draft conclusions on identification of customary international law, with commentaries, International Law Commission, available at: https://legal.un.org/ilc/texts/instruments/english/commentaries/1_13_2018.pdf
* ↑ https://www.ejiltalk.org/wp-content/uploads/2019/02/US-Views-on-ILC-Draft-Conclusions-on-CIL.pdf
* ↑ Daugirdas, Kristina. "International Organisations and the Creation of Customary International Law". European Journal of International Law. 31 (1).
* ↑ Kammerhofer, Jörg; Arajärvi, Noora; Merkouris, Panos, eds. (2022), "The Practice of Customary International Law Across Various Fora: Diversity of Approaches and Actors", The Theory, Practice, and Interpretation of Customary International Law, The Rules of Interpretation of Customary International Law, Cambridge: Cambridge University Press, pp. 229–344, ISBN 978-1-316-51689-8, retrieved 2023-05-30
* ↑ Baker, Roozbeh B. "Customary International Law in the 21st Century: Old Challenges and New Debates". European Journal of International Law. 21 (1): 177.
* ↑ Case concerning the Right of Passage over Indian Territory (Portugal v. India), Merits, Judgment of 12 April 1960: I.C.J. Reports, p. 44, available at: https://www.icj-cij.org/sites/default/files/case-related/32/032-19600412-JUD-01-00-EN.pdf
* ↑ Military and Paramilitary Activities in and against Nicaragua (see footnote 663 above), at p. 105, para. 199.
* ↑ Niels Petersen, The International Court of Justice and the Judicial Politics of Identifying Customary International law, The European Journal of International law, 2017, Vol. 28, no. 2, 366
* ↑ Continental Shelf, Dissenting Opinion of Judge Tanaka, available at: https://www.icj-cij.org/public/files/case-related/52/052-19690220-JUD-01-08-EN.pdf