The Dow Jones Industrial Average lost 22.6% on October 19, 1987, during the world's largest one-day percentage crash.
The session became known as Black Monday. Selling spread through markets worldwide, and the Dow's percentage decline exceeded every previous one-day fall in the index. The drop was especially striking because it occurred in a mature, heavily traded market rather than during a conventional banking panic.
Several explanations were debated, including high valuations, trade and interest-rate concerns, portfolio insurance, and computerized trading strategies. No single cause fully explains the event. Afterward, exchanges introduced or strengthened circuit breakers and other procedures designed to slow trading during extreme moves.
The 22.6% figure is a percentage change in the Dow, not a point loss. Point losses depend on the index's level, so later crashes can produce larger numerical point declines while remaining smaller percentage events.