Barter is the oldest system of trading where goods and services are exchanged directly without money. Under a barter system, there must be a "double coincidence of wants" where both traders have something the other wants in order to complete a trade. Other problems with barter include a lack of a common store of value as some goods are perishable, a lack of divisibility as commodities cannot always be divided into small units, and a lack of a common measure of value to assess the worth of different commodities. The difficulties of barter led to the invention of money as a common medium of exchange.