Purpose – Haldane has suggested that modularity would add sustainability to the financial system. The purpose of this paper is to suggest a route by which such modularity might be achieved. Design/methodology/approach – The paper attempts to explore the micro-foundations of regulatory regimes as rule bound orders and demonstrate that externally imposed rules may not be absolutely necessary to constrain the behaviour of individuals or organisations. Voluntarily self-agreed rules may allow for greater communication and monitoring among the participants in a group. This in turn can result in greater sustainability. The paper uses examples from the work of Ostrom on sustainable common-pool resources to support this view. Examples are also given from the financial services industry. Findings – The paper suggests that non-legislative, informal rules of behaviour may be a useful source of constraining unsustainable behaviour in the financial services industry. In turn these self-enforcing rule-bound regimes may facilitate one feature of sustainable systems – modularity. Practical implications – The paper suggests that stakeholders in financial systems may find it useful, on a bottom-up basis, to facilitate the creation of groups of financial institutions that would create and then adhere to self-enforcing rules that could result in sustainable practices. Originality/value – The originality of the paper is on the focus on self-created and self-enforced rule-following and on using the work of Ostrom in a financial services setting.