In trade, barter is a system of exchange in which participants in a transaction directly exchange goods or services for other goods or services without using a medium of exchange, such as money. Barter is considered one of the earliest systems of economic exchange, used before the invention of money. Economists usually distinguish barter from gift economies in many ways; barter, for example, features immediate reciprocal exchange, not one delayed in time. Barter usually takes place on a bilateral basis, but may be multilateral. In most developed countries, barter usually exists parallel to monetary systems only to a very limited extent. Market actors use barter as a replacement for money as the method of exchange in times of monetary crisis, such as when currency becomes unstable or simply unavailable for conducting commerce.