Which U.S. market index fell more than 20% on October 19, 1987?

The Dow Jones Industrial Average fell more than 20% on October 19, 1987.

The Dow dropped 508 points, or 22.6%, in a single trading session. The event became known as Black Monday and remains the largest one-day percentage decline in the index’s history. Stock markets in other countries also suffered dramatic losses, making the episode an international crash rather than an isolated New York event.

The Dow is a price-weighted index of selected large U.S. companies. Because it contains only a small number of stocks compared with broader indexes, it is not a complete measure of the entire American market. The S&P 500 also fell heavily in 1987, but the famous record commonly cited belongs to the Dow.

The crash led regulators and exchanges to reconsider market safeguards. Circuit breakers were introduced to pause trading during extreme declines, giving investors time to process information and potentially reducing panic-driven feedback loops.