Rather than focusing on a single currency, purchasing power parity (PPP) measures the purchasing power of currencies between countries. As an example, think of a gallon of milk that costs $3 in the US and 30 pesos in Mexico. The PPP exchange rate would be $1 to 10 pesos. If the market exchange rate is different, then it deviates from PPP.  PPP estimates can be useful when comparing living standards and economic output from different countries. In less-serious terms, The Economist's Big Mac index explores the concept by comparing the cost of a Big Mac in different countries.