The article explores how to choose the best mix of currencies for a country's economy. It looks at a model with fixed prices and vertical trade, where one currency is used for imports and another for exports. The study shows that the ideal currency mix depends on how trade is structured. It also suggests that if a country decides to peg its currency to another, the choice should consider how other countries react to exchange rate changes. This could explain why some East Asian countries peg their currency to the Chinese RMB.