An early form of trade, the Gift economy, saw the exchange of goods and services without an explicit agreement for immediate or future rewards. A gift economy involves trading things without the use of money. Modern traders generally negotiate through a medium of exchange, such as money. As a result, buying can be separated from selling, or earning. The invention of money (and later of credit, paper money and non-physical money) greatly simplified and promoted trade. Trade between two traders is called bilateral trade, while trade involving more than two traders is called multilateral trade.