Which U.S. stock index fell more than 20% in one day on Black Monday in 1987?

The Dow Jones Industrial Average fell more than 20% in one day on Black Monday in 1987. On October 19, the index dropped 508 points, equal to 22.6% of its value, making the decline the largest one-day percentage fall in the Dow's history.

The Dow tracks 30 large U.S. companies and is price-weighted, meaning companies with higher share prices have greater influence than companies with lower share prices. That design makes it different from market-capitalization-weighted indexes such as the S&P 500.

The S&P 500 and Nasdaq Composite also suffered severe losses during Black Monday, so the Dow was not the only important measure affected. However, its 22.6% figure is the widely cited statistic associated with the crash. The market's rapid recovery afterward helped distinguish 1987 from 1929, when the financial damage persisted for years and coincided with a much broader economic collapse.