With the exception of the common currency, the euro, all members of the European Union are required to follow the rules and regulations of the economic union. To date, nineteen of the current twenty-eight members of the European Union have given up their national currencies. The incentive to adopt the euro is the belief that a common currency will increase prosperity and promote deeper integration and closer political ties. A common currency reduces costs by eliminating the need for travelers to change their money when they cross borders, and for businesses to keep multiple accounts or currencies for receipts and payments. A further benefit for business is that a common currency eliminates exchange rate risk. The most important trading partners for EU countries are other EU countries so the elimination of exchange rate fluctuations provides an added degree of stability in payments and receipts. In the 1990s, as the euro project was negotiated and developed, proponents also hypothesized that it would cause there to be more trade and investment between member countries and that the ties binding together EU countries would strengthen.