Which U.S. stock-market index fell 22.6% on Black Monday in 1987?

The Dow Jones Industrial Average fell 22.6% on Black Monday in 1987.

Black Monday occurred on 19 October 1987, when the Dow lost 508 points, or 22.61%, in a single trading session. It remains the largest one-day percentage decline in the index’s history. Stock markets in other countries also suffered steep losses, making the event an international crash rather than only a U.S. episode.

Several explanations were proposed, including market overvaluation, rising interest rates, economic worries, and computerized portfolio-insurance selling. No single cause fully explains the speed and scale of the decline.

The crash did not produce a depression comparable to the 1930s. The Federal Reserve, led by Alan Greenspan, signaled that it would provide liquidity to the financial system. Trading rules were later changed, including the development of circuit breakers designed to pause markets during extreme falls.