The Dow Jones Industrial Average lost about 12% on Black Tuesday, October 29, 1929.
The fall followed a period of intense speculation and buying on margin, in which investors purchased shares with borrowed money. As prices turned downward, margin calls forced many investors to sell, adding further pressure. The exchange recorded roughly 16 million shares traded that day, a record at the time.
The 12% figure describes one session, not the total decline of the entire crash. The Dow had already fallen substantially from its September peak, and it continued dropping after October 29. By July 1932, it had lost nearly 89% from its 1929 high.
A common mix-up is to use the 22.6% figure associated with Black Monday in 1987. That later crash was much steeper as a one-day percentage event, while the 1929 collapse had a longer economic aftermath.