Which stock-market index lost almost 23% on Black Monday in 1987?

The Dow Jones Industrial Average lost almost 23% on Black Monday, October 19, 1987.

The one-day fall was the largest percentage decline in the Dow’s history. Selling spread through markets around the world, making the event an international crash rather than an isolated American disturbance. The S&P 500 and Nasdaq also suffered severe losses that day.

Several factors were associated with the crash, including high share valuations, rising interest rates, trade tensions, and computerized portfolio-insurance strategies. Those automated strategies could accelerate selling when prices moved down, although no single explanation accounts for the entire event.

The crash differed from 1929 in an important way: the economy did not immediately enter a depression. The Federal Reserve supplied liquidity, and markets eventually recovered. To reduce the chance of another disorderly collapse, U.S. exchanges later introduced circuit breakers that can pause trading after unusually large moves. The Dow is a price-weighted index of 30 prominent U.S. companies, so its percentage decline is not the same as the total loss of every listed share.