Which U.S. stock index lost 12.8% on Black Tuesday in 1929?

The Dow Jones Industrial Average lost 12.8% on Black Tuesday, 29 October 1929.

The Dow was the leading U.S. stock-market barometer of the period, so its collapse became a powerful symbol of the Wall Street Crash. On Black Tuesday, roughly 16 million shares changed hands, a record at the time. The selling followed steep losses on Black Thursday and Black Monday earlier that month.

The crash reflected speculative buying, purchases made with borrowed money, weakening economic conditions, and declining confidence. The market did not fall in one continuous movement, but the October losses marked a decisive break from the exuberance of the late 1920s. The Dow eventually fell almost 89% from its September 1929 peak to its July 1932 low.

The crash helped deepen the Great Depression, although it was not its sole cause. The S&P 500 and Nasdaq did not yet exist in their modern forms, making the Dow the clearest answer for this date.