Trade involves the transfer of goods and services from one person or entity to another, often in exchange for money. Economists refer to a system or network that allows trade as a market. An early form of trade, barter, saw the direct exchange of goods and services for other goods and services, i.e. 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 letter 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.