The document discusses bimetallism, which is a monetary system where the monetary unit is expressed in terms of two metals, usually gold and silver, at a fixed ratio set by the government. Under bimetallism, coins made of both metals circulate simultaneously and are unlimited legal tender. The system aims to provide adequate money supply, price stability, and encourage foreign trade, but it can be undermined by factors like the inequality of market and mint ratios and the operation of Gresham's law. By the 20th century, most nations abandoned bimetallism and moved to paper currency systems managed by central banks.