Parity has special meanings in such fields as physics, math, medicine, genetics, and marketing. Back when the Soviet Union and the U.S. were opposing superpowers, there was often talk of parity in nuclear weapons between the two sides. We sometimes hear about parity between mental and physical health in insurance coverage, or parity in colleges' funding of men's and women's athletics. But parity may be most common in discussions of currencies. The exchange rate between two national currencies often changes every day, as each drifts higher or lower, and occasionally two similar currencies, such as the euro and the U.S. dollar, will achieve parity, but it rarely lasts long.