Which U.S. stock index fell 20.47% on Black Monday in 1987?

The Dow Jones Industrial Average fell 20.47 percent on Black Monday, October 19, 1987.

That percentage decline was the Dow's largest one-day percentage fall in its history. The index, often called the Dow, tracks 30 prominent U.S. companies and is price-weighted rather than weighted by total market value. Its 1987 plunge therefore became one of the most widely cited numerical measures of the crash.

The S&P 500 and Nasdaq Composite also dropped sharply that day, but they are different indexes with different construction methods. Another common mix-up is confusing a percentage fall with a point fall: the Dow lost 508.32 points in 1987, while 20.47 percent describes the relative size of the decline.