This article explains the major influence that choices over currency in multi-level political unions can have. This applies to the UK in relation to the EU, since the UK is a member of the EU without having adopted its common currency, and hypothetically could have applied to relations between Scotland and the UK had the 2014 referendum been for an independent Scotland. The choice of the currency as a key path-making decision has already set the UK on a path divergent from the path on which eurozone countries are set. This consideration may be used as an argument for assuaging the impact of leaving the EU by advocates of the UK leaving the EU, or at the opposite it might be used as an argument for the UK not leaving. Historical new institutionalism, revised neo-functionalism, and frameworks drawn from the multilevel governance (MLG) literature are employed to underpin the argument.