Which U.S. stock index lost 20.47% during the single-day plunge known as Black Monday in 1987?

The Dow Jones Industrial Average lost 20.47% during the single-day plunge known as Black Monday in 1987.

Black Monday occurred on October 19, 1987, when stock markets around the world fell sharply. The Dow’s percentage loss was its largest one-day decline in history, although the points lost were much smaller than some later crashes because the index level was then far lower.

Several forces contributed, including expensive valuations, rising interest rates, fears about economic growth, and computerized portfolio-insurance strategies that accelerated selling. International market declines also reinforced investor panic. The precise balance among these causes remains a subject of financial-history discussion.

The crash did not produce a Great Depression-style collapse. The Federal Reserve, led by Alan Greenspan, issued a statement supporting the financial system, and markets eventually recovered. Black Monday is also distinct from the 1929 Black Tuesday episode and from later percentage declines in individual shares.