Which index lost 22.6% on Black Monday, October 19, 1987?

The Dow Jones Industrial Average lost 22.6% on Black Monday, October 19, 1987.

The one-day percentage fall was the largest in the Dow's history, and the decline spread across major markets worldwide. Hong Kong markets fell first in the global sequence, followed by other Asian, European, and North American exchanges. The New York market's fall exceeded the percentage declines seen during the famous crash days of 1929.

Several explanations have been studied, including program trading, portfolio insurance, overvalued shares, monetary concerns, and international trade tensions. No single cause fully accounts for the event. The trading system also faced severe pressure from the enormous volume of sell orders.

The crash did not produce a depression like the 1930s downturn. The Federal Reserve, led by Chairman Alan Greenspan, supplied liquidity and stated that it was ready to support the financial system. Trading rules were later changed, including the development of circuit breakers designed to pause markets during extreme declines.