The S&P 500 fell 20.47% on October 19, 1987, matching the global Black Monday shock.
The S&P 500 tracks large U.S. companies and is weighted by market capitalization. On Black Monday, it suffered its largest one-day percentage decline in modern index history, even though the Dow’s fall is more commonly quoted in popular accounts. The crash affected stocks broadly rather than just a few companies.
The decline occurred amid concerns about interest rates, trade imbalances, valuations, and automated portfolio-insurance strategies. Selling cascaded through markets in the United States and abroad. The episode led regulators and exchanges to develop coordinated procedures for pausing trading during extreme price movements.
The S&P 500 should not be confused with the Dow Jones Industrial Average. The Dow lost 22.6% that day because it uses a price-weighted method and contains 30 stocks, while the S&P 500 represents a much broader large-company universe.