Since the 1990s, western, developed countries have moved away from rule-making and standard-setting in multilateral intergovernmental organisations, and have increasingly collaborated on those matters in clubs of developed countries, such as trans-governmental regulatory networks. Although clubs often generate rules or standards that affect developing countries, the latter have not had a voice in rule-making, resulting in a ‘participation gap’, for which clubs are being criticised. Against this background, I analyse a recent development that has largely gone unnoticed: Clubs have been integrating previously excluded developing countries. From small and exclusive clubs, they are growing into larger and more inclusive clubs. I call this trajectory of the past 70 years—the establishment of intergovernmental organisations, their increasing displacement in favour of clubs, and the recent reversion towards larger clubs—accordion governance. Like an accordion that expands or contracts as needed, so too have governance models and rule-making adjusted to changing conditions and preferences by becoming more or less inclusive. Focussing on club expansion, I address three questions: (1) How has participation—and the rules governing it—evolved over time? (2) Why are governments voluntarily sharing rule-making authority with new participants? (3) Can these reforms close the participation gap in international rule-making?