A petroeuro is a petroleum trade valued in the euro as opposed to the US dollar (a petrodollar). Trading of any natural resource, including petroleum, is controlled through trading partnerships involving both exporters and importers of the resource, in a defined marketplace, and through a trade agreement. The major countries holding petroleum reserves since the decline of US production are dominated by OPEC, and hence, OPEC may choose dollar, euro, yen, or any currency providing perceived advantage, politically or economically. Since the agreements of 1971 and 1973, OPEC oil is exclusively sold in US dollars. This created a permanent demand for dollars on the international exchange markets.