The period of globalisation has seen more and more of international and regional organisation. Setting up an organisation with a group of state entails a resolution to the following two questions: (1) How are votes to be allocated? (2) What aggregation rule is to be employed? International and regional organisations display some interesting differences in how they have approached these two questions choosing a regime. The power index framework offers a convenient method for analysing these constitutional differences. It may be linked with the basic framework in constitutional economics – Wicksell’s classic approach, which entails that players very much use their preferences for the power to act and the power to prevent action when deciding the regime to be employed.
Key words: international organisation, EU, group decisiveness, blocking power, absolute voting power, Penrose-Banzhaf framework.