Theories that invoke purchasing power parity assume that in some circumstances (for example, as a long-run tendency) it would cost exactly the same number of, for example, US dollars to buy euros and then to use the proceeds to buy a market basket of goods as it would cost to use those dollars directly in purchasing the market basket of goods. Essentially GDP PPP controls for the different costs of living and price level, usually relative to the United States Dollar, that would make an accurate depiction of a given nation's gross income.