The Sacagawea dollar was authorized by Congress in 1997 because the supply of Anthony dollars, in inventory since their last mintage in 1981, was soon expected to be depleted. These coins have a copper core clad by manganese brass. Delays in increasing Sacagawea dollar production led to a final 1999-dated mintage of Susan B. Anthony dollars. Dollar coins are used infrequently in general commerce. They used to be given as change by United States Postal Service (USPS) stamp vending machines, which created a relatively small but significant demand, but the USPS eliminated all those machines by 2011. They were also used in certain subway and public transit systems, such as the "T" in Boston.