In the field of international economics and finance, the concept of Purchasing Power Parity (PPP) has been among the most enduring. Absolute PPP (which is in its ultimate form) indicates that on condition that a common currency dominates prices, people could purchase the same commodity anywhere for the same price. The law of one price provides certain theoretical guidance for the concept, assuming that arbitrage in many types of commodities equalizes prices. After the gold standard collapse during World War I, Cassel (1922) suggested that relative gold parities could be restored by using PPP. He said that post-war exchange rates were set by countries in terms of PPP by making sure that the change in their post-war and pre-war exchange rates was equal to the difference between their post-war and pre-war inflation rates. Since then, PPP has been used by economists to set and predict exchange rates, in adjusting cross-country incomes to explain differences in prices, and as a foundation of models in international macroeconomics.