Which U.S. stock index suffered the largest one-day percentage loss in the 1929 crash?

The Dow Jones Industrial Average was the prominent U.S. stock index whose recorded 1929 crash losses included the market’s largest one-day percentage declines. On Black Monday, October 28, the Dow fell 12.8%, followed by a 12.3% fall on Black Tuesday, October 29.

The Dow was then a much smaller index than today, containing 30 major industrial companies. It was widely followed as a barometer of U.S. business conditions, although it did not represent the entire stock market. The S&P 500 did not yet exist in its modern form.

The 1929 collapse was measured in both points and percentages, and the two can be confused. The Dow’s point losses were smaller than later crashes because its numerical level was lower, but its percentage falls were enormous. Trading volume also reached unprecedented levels during the crisis.