Which US stock-market index suffered the largest single-day percentage loss during the October 1929 crash?

The Dow Jones Industrial Average suffered the largest widely cited single-day percentage losses during the October 1929 Wall Street Crash. On October 28, the index fell 12.82 percent, followed by a further 11.73 percent decline on October 29, known as Black Tuesday.

The Dow was then the principal barometer of American shares, and its dramatic fall became the clearest numerical symbol of the crash. The collapse followed years of speculation, heavy use of margin loans, rising share valuations, and weakening economic conditions. Bank failures and falling industrial production deepened the damage afterward.

The S&P 500 did not yet exist in its modern form; Standard Statistics introduced a 90-stock index in 1926, while the modern S&P 500 began in 1957. The crash itself did not single-handedly cause the Great Depression, but it helped undermine confidence and worsened financial stress.