Which US stock index fell 12.9% on 28 October 1929, one day before Black Tuesday?

The Dow Jones Industrial Average fell 12.9% on 28 October 1929, one day before Black Tuesday.

The session became known as Black Monday in accounts of the 1929 Wall Street crash. The Dow had already suffered a major decline on the preceding Thursday, but selling intensified on 28 October and continued into the following day. On 29 October, Black Tuesday, the Dow fell another 11.7%.

The crash followed years of rising share prices, heavy margin borrowing, and widespread confidence that prices would keep increasing. As prices turned downward, margin calls forced investors to sell, adding to the pressure. The crash was an important event in the transition to the Great Depression, though it was not by itself the sole cause of that economic catastrophe.

The Dow is a price-weighted index of selected US companies. It is not the same as the broader S&P 500, which was introduced later in its modern form and uses a different weighting method.