The mint parity theory explains how exchange rates between countries on the gold standard were determined. Under this system, currencies were defined by and convertible to a fixed quantity of gold. The exchange rate between two currencies adhering to the gold standard was based on the parity of the mint price of gold in the two countries. This ensured stability in exchange rates as long as countries maintained their commitment to gold convertibility at a fixed price. However, the gold standard broke down in the early 20th century, rendering the mint parity theory obsolete.