Which US market index fell 12.9% on October 28, 1929?

The Dow Jones Industrial Average fell 12.9% on October 28, 1929.

October 28 became known as Black Monday during the Wall Street Crash of 1929. The Dow’s decline was followed by an even larger percentage fall on October 29, commonly called Black Tuesday. Together, the two sessions marked a dramatic acceleration of the market collapse.

The crash followed years of rising share prices, widespread margin borrowing, and optimism about permanent prosperity. Once prices began falling, leveraged investors faced pressure to sell, which intensified the downward movement.

The Dow Jones Industrial Average was then composed of 30 large US companies, as it is today, although its constituents have changed many times. The S&P 500 did not yet exist in its modern form, so attributing the 1929 daily figures to that index is a common historical error.