Derivatives are financial instruments whose value is based on an underlying asset such as commodities, precious metals, currencies, bonds or stock indices. The two main types are forwards, which are private contracts, and futures, which are exchange-traded standardized contracts. Derivatives markets originated in the middle ages and the Chicago Board of Trade was the first formal exchange established in 1848 to trade agricultural commodity derivatives. The Chicago Mercantile Exchange was later established in 1874 and also trades futures on currencies, interest rates, stock indices and other commodities.