The supply of any currency is upward-sloping because as its price rises, holders of that currency can obtain other currencies more cheaply and will want to buy more imported goods and, therefore, will give up more of their currency to obtain other currencies. As with other commodities, the intersection of the supply and demand curves for a currency will determine the price of exchange rate. Since, U.S. is economically and politically stable, then some other countries will prefer to have their savings into U.S. assets and, therefore, will increase the demand for dollars. If U.S. real interest rates increase relative to the rest of the world, then international investors will demand more for dollar dominated assets and therefore will increase the demand for dollar. This will lead to dollar appreciation.
Speculation is another determinant.
The Gold Standard was the international system used during 1879-1934 period. Many nations were using gold standard system during that period. This means that all nations fix their exchange rates in terms of gold. In this system, each nation must define its currency in terms of a quantity of good, maintain a fixed relationship between its stock of gold and its money supply, and allow gold to be freely exported and imported. The Bretton Woods system was enacted following World War II by treaty among the leading industrialized western nations.