The S&P 500 fell 20.47% on October 19, 1987, during the global crash known as Black Monday.
The decline was the largest one-day percentage fall in the index’s history. Selling began in markets outside the United States and intensified after the New York market opened. Analysts have linked the crash to high valuations, rising interest rates, trade tensions, portfolio-insurance strategies, and automated selling, although no single explanation accounts for every part of the event.
The S&P 500 tracks 500 large U.S. companies and is weighted by float-adjusted market capitalization. It differs from the Dow Jones Industrial Average, which contains 30 companies and is price-weighted. The crash also affected stock exchanges worldwide, but markets recovered more quickly than during many earlier crashes. The Federal Reserve, led by Alan Greenspan, issued a statement emphasizing its readiness to provide liquidity to the financial system.