From Wikipedia:

"The chief features of the Bretton Woods system were an obligation for each country to adopt a monetary policy that maintained the exchange rate of its currency within a fixed value—plus or minus one percent—in terms of gold and the ability of the IMF to bridge temporary imbalances of payments. In the face of increasing strain, the system collapsed in 1971, following the United States' suspension of convertibility from dollars to gold. This created the unique situation whereby the United States dollar became the "reserve currency" for the nation-states which had signed the agreement.