One of the early forms of trade, the free economy, saw the exchange of goods and services without explicit agreement on direct or future rewards. The free economy involved the exchange of things without the use of money. Modern merchants generally negotiate by some medium of exchange, such as money. As a result, it is possible to separate buying from selling, or earning. The invention of money (and later bank credit, fiat currency, and digital currency) greatly simplified and enhanced commerce. Trade between two merchants is called bilateral trade, while trade that involves more than two merchants is called multilateral trade.