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.  Traders generally negotiate through a medium of credit or exchange, such as money. Though some economists characterize barter (i.e. trading things without the use of money[1]) as an early form of trade, money was invented before written history began. Consequently, any story of how money first developed is mostly based on conjecture and logical inference. Letters of credit, paper money, and non-physical money have greatly simplified and promoted trade as buying can be separated from selling, or earning. Trade between two traders is called bilateral trade, while trade involving more than two traders is called multilateral trade.