the Bretton woods agreement of 1944 established fixed exchange rates refined in terms of gold and the U.S dollars.
between 1944 and 1971, many currencies were pegged against the U.S. dollar, I.e their parities with the U.S. dollar were fix.
in this period, a U.S. dollar was promissory note issue by the United States treasury.
if anybody requested it, the treasury has no exchange the note for 1/35th of an ounce of gold.