Which U.S. index fell about 20 percent in a single day during the 1987 Black Monday crash?

The Dow Jones Industrial Average fell about 20 percent in a single day during the 1987 Black Monday crash.

More precisely, the Dow declined 22.6 percent on October 19, 1987. It was the largest one-day percentage fall in the index’s history and remains a defining statistic of modern financial markets. The Dow represented 30 major U.S. companies and was widely watched as a measure of market confidence.

Other U.S. indexes also dropped heavily, including the S&P 500 and Nasdaq Composite. The scale of the selling reflected international contagion, investor anxiety about economic conditions, and automated trading systems that could accelerate declines.

A frequent mistake is to compare the 1987 loss only in points. The Dow’s point level was much lower than it is today, so the percentage loss gives the clearest historical comparison. Unlike the 1929 crash, Black Monday did not lead to a decade-long depression, partly because policymakers acted to maintain financial liquidity.