President Richard M. Nixon enacted sweeping price and allocation controls in response to the 1973 oil boycott. Oil could be sold at different prices, depending on when it had been found. States were allocated oil in 1974 based on what they had used in 1972. Colored flags indicated if rationing was in effect or if no gas was available. Most memorable, though, were the odd-even gas days: Drivers with an odd number as the last digit of their license plate could buy gas one day, those with an even-numbered plate on the next. Vanity plates counted as odd.