5.1 Introduction

As part of the research theme of interpretation of unwritten international law, this chapter zooms in on instances of interpretation of customary international law (CIL) within an institutional context. The involvement of such a ‘context’ – here used to refer primarily to the institutional environment of an international intergovernmental organization – has different facets, which so far have not been systematically examined. The aim of this chapter is to provide an overview of the ways in which institutional context may be connected to the interpretation of custom and to offer a hypothesis on interpretative approaches that may emerge therefrom.

Interpretation in (international) law is generally taken to refer to a ‘process of assigning meaning to … statements for the purposes of establishing rights, obligations, and other consequences relevant in a legal context’.Footnote ¹ It is a commonplace that the interpretation of written international law is an elusive operation.Footnote ² This holds both for interpretation performed as an explicit act, for instance, by a court, and for interpretation performed as a non-formal and often implicit act by legal actors and stakeholders involved. The interpretation of unwritten or customary international law is still a more complex exercise, which makes it a timely topic of study.Footnote ³ International organizations then are frequently the setting for interpretation of customary international law, even when the interpretation is not conceived as a formal or even explicit act.

The picture of custom interpretation in an institutional context is multifaceted. For one, it may be at issue where international custom is applied – and perhaps has been co-created – at general international level by an international organization. It may also be found when inside an organization’s institutional order non-binding rules and norms are set (with resolutions of the UN General Assembly as a prominent example). Technically speaking, these rules may give rise to custom and ultimately become binding that way, to be subsequently interpreted when they are put to application, both by member states and by the organization and both inside and outside the organization’s institutional sphere. In some cases, the organization’s adoption of non-binding instruments such as ‘resolutions’, may as such amount to a form of interpretation, because of the resolution’s (implicit) claim to be declaratory of existing custom.Footnote ⁴

Interpretation of custom by courts and tribunals, even if these do have an institutional dimension, is in principle left out of the account in this chapter. Explicit acts of custom interpretation in a (semi-)judicial setting – eminently valuable for bringing concrete interpretative issues to a head – have already been examined comparatively extensively.Footnote ⁵ In order to capture also more implicit and fluid acts of interpretation, the present chapter focuses on (other) actors and institutional frameworks. This entails the use of a broad notion of ‘interpretation’, which essentially includes ‘application’; this in recognition of the notoriously thin line between ‘application’ and ‘interpretation’Footnote ⁶ (somewhat related to the dual meaning of ‘interpretation’ as CIL ascertainment and as CIL content determination),Footnote ⁷ as well as between ‘interpretation’ and ‘modification’ of rules.Footnote ⁸ It may be argued that such boundaries are still thinner for CIL than for written rules.

The chapter first addresses instances of organizations applying and interpreting customary international law on par with other international actors (Section 5.2), also considering the thorny subject of the contribution of organizations themselves to the formation of custom at international level (Section 5.3). Another main instance where ‘institutional context’ plays a role is when organizations or their member states apply and interpret unwritten international law within the institutional order (Section 5.4). A particular dynamic occurs when organizations produce within the institutional order acts of ‘soft law’, which plausibly in one way or another (co-)create customary international law, to be subsequently developed further through interpretation on the part of the organization and/or the member states (Section 5.5). The chapter then briefly considers the notion of ‘customary institutional law’ (Section 5.6) and proposes some considerations as to what interpretative trends may be surmised in relation to the institutional context (Section 5.7). It ends with concluding remarks (Section 5.8).

5.2 Organizations as Interpreters at International Level

One way in which an institutional context may be at play in interpretation is when international organizations as such apply and interpret general customary international law. A notable example is the domain of international human rights law (IHRL), with which the United Nations (UN) has engaged intensely over time, increasingly also with regard to its ‘own’ conduct. In the background, scholarly discussion continued to be animated by the question as to whether the UN, or any organization, could be considered bound by customary human rights law. The possibility of a customary basis for international human rights had been at issue already in the 1960s and 1970s and had culminated in an unsuccessful attempt in the context of General Comment 24 of the Human Rights CommitteeFootnote ⁹ to list human rights in the Covenant with customary status.Footnote ¹⁰ In its 2007 judgment in the Diallo case, the ICJ confirmed that international human rights have assumed customary status.Footnote ¹¹ Since the 2000s, rules of IHRL were essentially declared by the UN to be binding upon itself,Footnote ¹² and thereby also came to be interpreted by the organization. The field of international humanitarian law (IHL) shows a similar development, as evidenced by the 1999 bulletin of the UN Secretary-General setting out ‘fundamental principles and rules of international humanitarian law applicable to United Nations forces conducting operations under United Nations command and control’.Footnote ¹³

At the same time, the view that international organizations as such could not generate legally relevant practice or opinio juris was widely shared, as appears in a general sense from the travaux of the 1986 Vienna Convention dating from the same period.Footnote ¹⁴ The 2018 ILC Conclusions on the identification of customary international law, addressed below, have echoes of this outlook. They suggest that a binding effect of custom on organizations would be incidental, since the symmetry envisaged in traditional international law – legal actors can only be bound by obligations to which they have consented or by law they have co-created – would be lacking.

Meanwhile, the debate has been spurred, especially by developments in the field of human rights, and the proposition that international organizations are fully bound by relevant customary international law has not been controversial for at least two decades.Footnote ¹⁵ Still, while it is a well-received proposition, it is not always clear by what mechanism international organizations become bound by customary international law, including human rights law and humanitarian law.Footnote ¹⁶ According to one scholar ‘jus cogens norms always bind IOs and … general international law binds IOs as a default matter’.Footnote ¹⁷ Another author, after a careful examination of theoretical explanations of international organizations being bound by custom, and rejection of views that derive such bindingness in some way from the member states, opts for a constitutionalist (arguably ‘public’) view of international law, in which organizations are bound in their capacity of participants in the international order.Footnote ¹⁸

Either way, it is clear that such binding effect cannot be accounted for in the voluntarist and symmetrist reasoning of classic international law, as appears from the next section, which addresses the limitations posed to international organizations as lawmaking actors. This must mean that the proposition rests at least partly on a public law conception of international law: only then does it make sense to admit new members to a ‘legal community’ on condition they accept the legal ‘acquis’ of that community. Such a public law conception in fact points to a view in which all customary international law – including branches that do not focus on the human individual in the way of IHRL and IHL – would be binding on international organizations.

5.3 The Contribution of Organizations to International Custom

Thus, international organizations acting in accordance with customary law that is (construed as) binding upon them is one way in which ‘institutional context’ may be involved with the interpretation of custom. For a further understanding of such involvement, it is helpful also to consider the position of organizations from a reverse angle and look at the – fundamental – question of whether international organizations are able to produce custom or contribute to its formation. As mentioned above, the 2018 ILC Conclusions on the identification of customary international law suggest that contemporary international law and doctrine invariably accord a very modest role to international organizations as independent actors in the formation of customary international law.Footnote ¹⁹ This could seem remarkable, given that international organizations have an ever more prominent role in international legal life.

In the 2018 Conclusions, international organizations appear in two ways: as a platform for the (legal) acts of States and as an independent international-legal actor. Conclusion 4 (‘Requirement of practice’) para 2 is the only provision that refers to the practice of international organizations in their own right, as a possible contribution to ‘the formation, or expression, of rules of customary international law’. This is most likely to occur, according to the commentary – with a clear (and restrictive) reference to ‘regional (economic) integration organizations’ such as the EU – when States have transferred competences to an international organization or ‘conferred powers upon the international organization that are functionally equivalent to the powers exercised by States’.Footnote ²⁰ The opinio juris of organizations is not mentioned, although the commentary states that para 2 of Conclusion 10 (‘Forms of evidence of acceptance as law’) is applicable mutatis mutandis.Footnote ²¹ More generally, the ILC commentary points to a key role for states – where it should be noted that the Reports of the Special Rapporteur in respect of organizations take a more nuanced approach, referring, for example, to ‘operational activities’ of organizations that may amount to ‘relevant practice for purposes of formation and identification of customary international law’.Footnote ²²

Elsewhere I have proposed one explanatory factor for the limited role of organizations in the formation of custom. This is connected to the necessity in international law to ascribe to organizations a ‘juridical will’ – a term used as shorthand for the various subjective ‘intentional states’, such as ‘intention’, ‘opinion’, ‘acquiescence’, ‘consent’, that systems of law project onto legal participants in order to operationalize their legal agency.Footnote ²³ International law seems to have (since the nineteenth century) no problem attributing such legally relevant ‘psychological states’ to a state but traditionally shows signs of hesitation when it comes to an international organization. This in turn can be linked to a functionalist, mechanistic view of organizations’ legal personality, subservient to states as the original legal persons and prime political actors.Footnote ²⁴ It is one reason why the ascertainment of an organization’s opinio juris, or of an organization’s distinct ‘will’ for the purpose of attributing a particular practice, can pose a challenge. It could certainly also stifle the position of organizations as interpreters of CIL at global level.

5.4 Interpretation of Custom inside the Institutional Framework

Another instance of interaction between custom interpretation and institutional context is found when customary rules are interpreted within the institutional framework of an organization. One example is the strong role in (customary) lawmaking processes by the UN High Commissioner for Refugees (UNHCR). It has been argued before that the slow development of treaty-based international refugee law has created a need for more informal ‘soft law’ standards to supplement the provisions found in the 1951 Convention.Footnote ²⁵ Research shows how the UNHCR has filled that gap, interpreting and to a limited extent shaping the obligation of non-refoulement.Footnote ²⁶ This occurs especially through its Executive Committee, which annually adopts by consensus ‘Conclusions on International Protection’. The conclusions are not binding upon states, but they are conceptualized (with an implicit interpretation-claim) as ‘expressions of opinion which are broadly representative of the views of the international community’Footnote ²⁷ and exercise a steady normative influence.Footnote ²⁸ The UNHCR gives evidence of influence on refugee law in several respects, as is pointed out by Trevisanut,Footnote ²⁹ and this is not just about interpretation of written rules. Non-refoulement, which became interlinked with the law of the sea, has long since been ascribed also customary status. This is expressed, for instance, in a UNHCR amicus curiae brief, in which the Agency addresses the geographical remit of the principle of non-refoulement and specifically its application at sea. The brief explicitly draws on ‘the broader human right of refugees to seek asylum from persecution as set out in the Universal Declaration of Human Rights’.Footnote ³⁰

For one who adopts a legal lens, it is not always evident how to distinguish interpretation by the organization from that of the member states. That said, it is undisputed that inside an institutional framework both the organization and the individual member states may engage with rules of custom, sometimes simultaneously. Individual states’ voting behaviour in the UN General Assembly, or inside the institutional framework, is a familiar element in the ICJ’s reasoning on custom, as can be gleaned from the 1986 Nicaragua Judgment and the 1996 Legality Opinion.Footnote ³¹ Further deconstruction of an institutional procedure can be found in the 2011 dispute between the Former Yugoslav Republic of Macedonia (FYROM) and Greece.Footnote ³² When at the 2008 NATO summit Greece blocked FYROM’s bid to become a member of NATO, FYROM claimed that by putting in a negative vote, Greece had breached its obligations under Article 11 of the 1995 Interim Accord between Greece and FYROM. In order to examine possible legal responsibility on the part of Greece for its conduct within NATO, the International Court ‘pierced’ the institutional veil of the organization and found in favour of the claimant. Commentators spoke of ‘the first time in which the ICJ so assertively identified the individual conduct of a State in the process of institutional decision-making.’Footnote ³³

Looking at states, it can moreover be difficult to distinguish between state (interpretative) acts ‘inside’ an international organization and those ‘outside’ of it. No doubt the – porous – boundary between the institutional order of an organization and the general international legal order is ‘real’ in a legal-doctrinal sense. This is borne out by straightforward examples such as Article 5 in the 1969 Vienna Convention on the Law of TreatiesFootnote ³⁴ (i.e. on how the law of treaties applies to treaties adopted within an institutional framework but ‘without prejudice to any relevant rules of the organization’) and Article 64 of the 2011 Draft Articles on the Responsibility of International OrganizationsFootnote ³⁵ (on the ‘rules of the organization’, which as lex specialis would remain outside the scope of the Draft Articles). That said, especially with the (in the words of the ILC) ‘holistic’ exercise of interpretation, it can be unclear whether particular state conduct is part of – in legal terms – an institutional process or a global process. The distinction between the ‘internal’ institutional level and the general international level may be obfuscated all the more when these, in terms of membership, practically overlap, as in the case of an organization with universal membership such as the UN.

This is in addition to the above-mentioned fluidity of categories within the institutional framework, where it can be unclear whether an interpretative act must be attributed to the organization or to individual member state(s). In all cases, it seems logical to start from the ‘constitutional limit’ of organizations or, to put it differently, the principle of speciality,Footnote ³⁶ to determine whether a particular organization as such is even suited to engage with a particular rule or norm. Otherwise, law and practice do not offer clear guidelines in respect of these fuzzy boundaries, which means that sometimes both the organization and the member states will be interpreting the same rule of custom. In a socio-legal vein, who can successfully claim to be the prime interpreting actor might depend on how they have negotiated authority. The shaping and solidifying of the customary ‘human right to water’, which has changed substance and scope several times over the past twenty years, may serve as an example. Apart from classic diplomatic work by states, such as by the international watergezant on behalf of the Netherlands,Footnote ³⁷ the process has involved interpretation within the UN context by institutional bodies (examples are the 2002 General Comment 15 of the ICESCR Committee,Footnote ³⁸ the 2008 Human Rights Council resolution on human rights and access to safe drinking water and sanitation,Footnote ³⁹ and the (not unanimously adopted) 2010 UN General Assembly Resolution on the human right to water and sanitation)Footnote ⁴⁰ and by states (examples are the ‘acceptance speech’ regarding the human right to water and sanitation by the Netherlands in Geneva in 2008Footnote ⁴¹ and individual abstentions by prominent states such as Canada when voting for the 2010 Resolution in the UN General Assembly).Footnote ⁴²

5.5 Normative Acts Produced within the Organization

The institutional context has an especially distinct role in case of the adoption, within an organization of a non-binding act (binding acts are left aside as they would call strictly speaking for the interpretation of written rules). Many ‘soft law’ acts will not give rise to custom, but some of them will. In the latter regard, a preliminary consideration is the classic requirement of a ‘fundamentally norm-creating character’,Footnote ⁴³ to determine whether the normative and prescriptive content of the act is of a suitably general nature. The role of UNGA Resolutions in the formation of customary international law was hotly debated already in the 1970s.Footnote ⁴⁴ The aforementioned ILC Conclusions on the identification of customary international law are a recent pronouncement in this delicate area of international law and doctrine. The Conclusions provide that ‘a resolution adopted by an international organization … may provide evidence for determining the existence and content of a rule of customary international law’.Footnote ⁴⁵

The relevance for custom interpretation is shown by certain UN General Assembly resolutions, which, as instruments, famously constitute ‘soft law’ and as normative substance are then recognized to have crystallized into customary international law. Now, how do non-binding, arguably CIL-creating resolutions of the UN General Assembly fare when it comes to interpretation – such as the 1960 Declaration on the Granting of Independence to Colonial Countries and PeoplesFootnote ⁴⁶ or the 1970 Declaration on Principles of International Law concerning Friendly Relations and Cooperation among States in accordance with the Charter of the United Nations,Footnote ⁴⁷ to name two classics? A solid answer to this question requires separate research. But there is no doubt that the resulting rules and norms are subject to some form of interpretation, be it a ‘by proxy’ textual interpretation (which is readily facilitated by the ample production of text in an institutional context – see below) or a ‘logical interpretation’.Footnote ⁴⁸ Neither is there any doubt that such interpretation can involve both organizations and states, as is apparent for instance in the Chagos Advisory Opinion when the ICJ in relation to the ‘maximalist interpretation of self-determination’Footnote ⁴⁹ based on resolution 1514 (XV) routinely refers to the considerations of both ‘the General Assembly or any other organ of the United Nations’ and of ‘States [who] have consistently emphasized … respect for the territorial integrity of a non-self-governing territory’.Footnote ⁵⁰

In relation to the interpretation of UN General Assembly resolutions, it is otherwise worthwhile to point to the powerful dynamic of contestation and confirmation, which is perhaps not found in regular international relations. This holds equally for other international organizations and their decision-making organs, especially when these are constituted by state representatives. The public records, public voting and tracing of numerical majorities and minorities make for a comparatively proceduralized and stylized process of custom formation and interpretation. This has, for instance, allowed us to follow the – in terms of normative content and customary legal status – hapless fate of the agenda of the ‘New International Economic Order’ that was importantly brought to the fore in the 1974 UNGA Resolution.Footnote ⁵¹ Also the development of the concept of Responsibility to Protect can be traced along a string of soft law instruments, from the ‘launch’ of the concept in 2001 by an external report commissioned by the UN Secretary-General (which left a small opening for the possibility of unilateral humanitarian intervention) to the 2005 UN ‘Outcome Document’ (in which the concept had been re-interpreted so as to align entirely with the collective security system of the UN).Footnote ⁵² Already in the 1940s, a seminal piece was written on the (mostly legal) statements by the UN Secretary-General and their influence on international law.Footnote ⁵³

The 2015 UNGA Resolution promulgating the Sustainable Development Goals,Footnote ⁵⁴ too, is well-placed to generate custom. The intricate process of application and interpretationFootnote ⁵⁵ of the resolution will, interestingly, differ somewhat from classic examples because the normative framework is largely operationalized at the states’ domestic level, through a complex system of indicators (themselves the subject of continuous discussion) and reporting on performance and implementation.

A final example to mention concerns the recommendations of the Universal Periodic Review organized by the Human Rights Council. Unlike the pronouncements and interpretative efforts of (semi-)judicial bodies, the Universal Periodic Review process is not based on a concrete complaint but on member states’ reciprocal recommendations, involving in turn application and interpretation of human rights. As is set out in a fine analysis by Cowell, the ‘[r]ecommendations are numerous and wide ranging but, when aggregated, can demonstrate certain trends in relation to human rights protection’.Footnote ⁵⁶

It has been pointed out that customary international law as a discourse poses a foundational(ist) problem since its grounding is ‘a past social reality’ that ‘can never be traced but can solely be presupposed’.Footnote ⁵⁷ This question emerges also when custom is considered from an immanent, positive law-perspective. Here too, in a different guise, the interpretation of unwritten law is an exercise that rests on shaky ground, because no ‘objective’ text exists but only a proposed rule that is itself an interpretation – thus posing an interpretative challenge akin to that of infinite regress. While it may be true that in one way or another ‘CIL is always shrouded in a “lexical garment”’,Footnote ⁵⁸ especially an environment which produces a considerable amount of written ‘soft law’ or ‘informal law’ – such as an international organization – may at some level bypass this infinite regress problem, as it creates text. Not only is it fair to say that some (non-binding) resolutionsFootnote ⁵⁹ are interpretations, especially where they claim to be declaratory of custom. It is also recognized that (written) re-statements of unwritten law (the Customary IHL Database of the ICRC is a prime example)Footnote ⁶⁰ have a ‘transformative’ dimension.Footnote ⁶¹

5.6 Customary Institutional Law

Focusing on substance, we see that the previous sections deal with ‘general international customary law’. Next to this, a special category of ‘institutional law’ exists within the framework of every international organization. The domain of institutional law comprises both institutional acts with ‘external’ effect (that is, directed at the member states) and institutional acts with ‘internal’ effect (that is, directed at the administration and operation of the organization as such).Footnote ⁶² In principle, ‘customary institutional law’, either within a single organization or as some scholars would have it, shared by multiple organizations (thus amounting to the elusive ‘common law of IOs’),Footnote ⁶³ can come into existence through the same process of interpretation and application.

We frequently find application and interpretation of customary institutional law in the UN context. An example is the unwritten rules – in origin related to the UN Charter – concerning the voting procedure in the UN Security Council (UNSC), which were then applied and interpreted further by the Council in the 1950s when abstention from voting by a permanent member came to be interpreted in such a way that it did not amount to a negative vote and hence would not trigger the ‘veto’.Footnote ⁶⁴ Another example is the continuously developing body of customary institutional rules and legal classifications in regard of the phenomenon of ‘peacekeeping’ that famously has no basis in the UN Charter.Footnote ⁶⁵ More recent re-interpretation of unwritten institutional rules is found with the UN General Assembly. Its practice of decision-making by consensusFootnote ⁶⁶ dates to the 1960s and was itself for a long time based on UN institutional custom, unmentioned in the Rules of Procedure.Footnote ⁶⁷ Interpretation and application of that unwritten rule then produced a new sub-category, the procedure entitled ‘adoption without a vote’. In this case, the UNGA President asks the Assembly if it wishes to adopt a draft resolution or decision. If no objection is made, the president declares ‘it is so decided.’ But this decision, unlike a ‘decision by consensus’, is not followed by a proclamation that ‘consensus has been reached’ and, importantly, (according to current practice) states do not give public explanations of their position.Footnote ⁶⁸ In recent years, this formula has come to be used for politically weighty resolutions.Footnote ⁶⁹

5.7 Interpretative Trends

Can any interpretative trends be inferred from this cursory overview? Clearly, further research is needed for substantiated findings. It is, however, fair to mention that at first blush the examples recounted in this text, especially when these relate to interpretation within the institutional framework of an organization, suggest an alignment of the interpretative exercise with the ‘objectives’ of the organization. This would match a prime characteristic of treaty interpretation in an institutional context, as has been examined notably in relation to constitutive treaties.Footnote ⁷⁰ Because of that match, and also because courts have been found to have recourse to rules of treaty interpretation when interpreting custom,Footnote ⁷¹ it is relevant briefly to consider interpretation of treaties in an institutional context.

In the 1996 WHO opinion, the International Court of Justice stated that ‘the imperatives associated with the effective performance of [the organization’s] functions as well as its own practice’ deserved ‘special attention’ in the interpretation of constitutive treaties.Footnote ⁷² How is this ‘special attention’ brought to bear? Traditionally, treaty interpretation accords considerable weight to the parties’ consent as a basis for the treaty. Articles 31 and 32 of the VCLT thus emphasize a ‘textual’ approach, whether alone or in concert with an ‘intentional’ approach. A similar method may be gleaned from the ICJ’s practice. Conversely, ‘object and purpose’ as an interpretative tool to establish the text’s meaning receives less attention.Footnote ⁷³

In contrast, the interpretation of a constitutive treaty – which embodies the institutional context in that it grounds and sets out the structure of the organization – appears to have been generally inspired by the treaty’s object and purpose or, put differently, by the principle of effectiveness in connection with the objective of the organization.Footnote ⁷⁴ When it comes to organizations’ (binding) secondary law, taken here as extended treaty law, the interpretative practice appears somewhat more restrained than the interpretation of constitutive instruments, likely due to ‘the delegated character of secondary rules’.Footnote ⁷⁵

Overall, one trend in the interpretation of constitutive treaties that stands apart from standard practice is the ‘teleological approach’ to the text, similar to traditions of statutory interpretation in a domestic, constitutional context. Here, some terminological distinction is helpful: the familiar notion ‘object and purpose’ is used in Article 31 VCLT to elucidate a textual approach for discerning the ‘ordinary meaning’ to be given to a treaty’s terms.Footnote ⁷⁶ This is not quite the same as a ‘teleological approach’, which in its classic sense is a general interpretative approach, taking the treaty’s objective as a guiding principle for an ‘effective’ interpretation of the text.Footnote ⁷⁷ Both approaches play a role in the interpretation of constitutive treaties, but the teleological approach is most prominent. It is also the most relevant for the interpretation of custom because it is the least text-bound.

Why teleological treaty interpretation shows signs of prevalence in an institutional context is not entirely clear. One plausible factor is that organizations have a vertical dynamic, as was captured early on by the notion of ‘centralization’ used by Kelsen in relation to the United Nations.Footnote ⁷⁸ Another factor may be that each organization represents a single epistemic community, or ‘interpretive community’, similar to a state domestic legal order governed by a single constitutional instrument.Footnote ⁷⁹ The existence of an interpretative community, reinforced by the ‘sectoral constitutionalization’ in an organization,Footnote ⁸⁰ is a powerful influence – even if of course it is not a simple dynamic. Similar to different branches in public law professions (the legal advisor and the judge, to name two),Footnote ⁸¹ different organs within an organization are known to form distinctive interpretative communities: think of the ‘three UNs’Footnote ⁸² or of the well-known contrast between the UN sector working on Peace and Security (‘UN New York’) and the UN sector involved in Human Rights work (‘UN Geneva’).

The hypothesis put forth in this chapter is that the institutional environment conducive to a teleological interpretation of treaty law also has this effect on the interpretation of customary international law.

5.8 Conclusions

The aim of this chapter is to provide a bird’s-eye view of the scenarios in which custom interpretation is connected with an institutional context – a frequent occurrence. Insight into the specificities and intricacies of these scenarios is therefore an essential tool for our understanding of custom interpretation.

Institutional context can play a role in custom interpretation in two main ways: an international organization may engage in custom interpretation alongside states at the general international level. Secondly, custom may be interpreted by the organization or the member states inside the institutional framework – for instance, through deliberations or decisions of an organ. A striking phenomenon is the production of what this chapter has termed ‘written custom’: non-binding acts adopted by an organization, which give rise to custom that, in turn, is applied and interpreted within (and outside) the framework of the organization.

A key proposition of this chapter is that the steady production of non-binding written instruments makes international organizations a dynamic normative environment with a thriving practice of – mostly implicit – custom interpretation. As argued, this interpretative practice is both relevant and understudied. Hence, the practice of custom interpretation in an institutional context deserves scholarly examination alongside the outstanding research that is focused on explicit efforts of custom interpretation carried out by courts, tribunals and semi-judicial bodies.

As this chapter sets out, the ‘institutional context’ has a number of complexities to be taken into account. Two may be reiterated here. First, the separate institutional sphere of an organization adds a layer of normativity and authority next to that of general international law. Therefore, it may happen that written rules created by the organization would have normative effect within the institutional framework but not per se in general international law (such as the Operational Policies and Procedures of the World Bank). Conversely, a rule of customary international law could be of uncontested validity but not automatically have normative force within the institutional framework.

Second, within the institutional framework, conduct of the organization may not be readily distinguishable from the conduct of member states. As confirmed in the ILC’s work on custom identification, in some situations the question arises as to which entity actually is performing the interpretation. The answer may have implications for the authority and the embedding of the interpretative act. That said, international (institutional) law does not for now offer clear guidelines on this point – with the arguable exception of the ‘principle of speciality’.

As for patterns of interpretation, the introductory overview in this chapter does not allow for solid conclusions. The examples do, however, suggest an orientation on the objective(s) of the organization and/or their expression in IO ‘soft law’ acts. In other words, they point to a predominantly teleological approach. This leads to another key proposition of this chapter, which is that the institutional context apparently conducive to a teleological interpretation of treaty law generally has the same effect when it comes to the interpretation of custom.

For the purpose of this chapter, ‘institutional context’ is defined as the institutional sphere of an international organization. However, we know that in relation to treaty interpretation the ‘institutional effect’ is also found in less formal and more fluid institutional arrangements, such as treaty regimes with a ‘constitutional character’ (possibly featuring a compliance mechanism or a mechanism for subsequent decisions by treaty parties).Footnote ⁸³ Insofar as these regimes engage with custom, it is a reasonable expectation that the ‘institutional effect’ would work in the same way.

The current chapter takes the technical-legal dimension of ‘institutional context’ as a starting point, from the idea that this can contribute to the analytical clarity in a legal examination of custom interpretation. But more angles are possible. With the legal landscape becoming ever more dense, a focus on the politics of interpretation appears ever more important. As illustrated by the aforementioned process of interpretation of the ‘responsibility to protect’, the multilayered and proceduralized environment of an international organization such as the UN brings out the politics of interpretation much more clearly than a classic international law environment.