Bimetallism, also known as the bimetallic standard, is a monetary standard in which the value of the monetary unit is defined as equivalent to certain quantities of two metals, creating a fixed rate of exchange between them. In all known historical cases, the metals are gold and silver.
For scholarly purposes, "proper" bimetallism is sometimes distinguished as permitting that both gold and silver money are legal tender in unlimited amounts and that gold and silver may be taken to be coined by the government mints in unlimited quantities. This distinguishes it from "limping standard" bimetallism, where both gold and silver are legal tender but only one is freely coined (e.g. the monies of France, Germany, and the United States after 1873), and from "trade" bimetallism, where both metals are freely coined but only one is legal tender and the other is used as "trade money" (e.g. most monies in western Europe from the 13th to 18th centuries). Economists also distinguish legal bimetallism, where the law guarantees these conditions, and de facto bimetallism, where gold and silver coins circulate at a fixed rate.