The single-day fall that made October 19, 1987, famous was a 22.6% drop in the Dow Jones Industrial Average.
The date became known as Black Monday. The Dow lost 508 points, then the largest one-day point decline in its history, as selling spread through markets in Asia, Europe, and North America. The decline was intensified by computerized trading strategies, portfolio insurance, high valuations, and concerns about interest rates and the dollar.
The 1987 crash differed from 1929 in important ways. The economy did not enter a depression, and the U.S. market eventually recovered. Regulators later introduced circuit breakers, which can temporarily halt trading after unusually large declines. The 22.6% figure refers specifically to the Dow’s percentage loss on that day, not to every stock exchange worldwide.