Trade involves the transfer of the ownership of goods or services from one person or entity to another in exchange for other goods or services or for money. Possible synonyms of "trade" include "commerce" and "financial transaction". Types of trade include barter. A network that allows trade is called a market. The original form of trade, barter, saw the direct exchange of goods and services for other goods and services. Later one side of the barter started to involve precious metals, which gained symbolic as well as practical importance. 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 greatly simplified and promoted trade. Trade between two traders is called bilateral trade, while trade between more than two traders is called multilateral trade. Trade exists due to the specialization and division of labor, in which most people concentrate on a small aspect of production, trading for other products.