The cost of changing one currency into another is known as the exchange rate. Currency is typically associated with a certain country. They might be subnational, like Hong Kong's currency, or international, like the euro. Each country has its own exchange rate mechanism for its currency. Governments have the power to impose restrictions and rules on currency rates. Additionally, countries' currencies may be strong or weak. Economists cannot agree on the appropriate national exchange rate (unlike on the subject of trade, where free trade is considered optimal). Political forces, on the other hand, impact national exchange rate regimes.