Explain the exchange rate system that existed during the 1950s and 1960s. How did the Smithsonian Agreement

Question:

Explain the system that existed during the 1950s and 1960s. How did the Smithsonian Agreement in 1971 revise it? How does today's system differ?
Exchange Rate
The value of one currency for the purpose of conversion to another. Exchange Rate means on any day, for purposes of determining the Dollar Equivalent of any currency other than Dollars, the rate at which such currency may be exchanged into Dollars...