The Dow Jones recorded its greatest point loss of that period on 29 October 1929, known as Black Tuesday.
The index lost about 12% that day as millions of shares changed hands in a wave of panic selling. Black Tuesday followed Black Thursday on 24 October and Black Monday on 28 October. Together, these sessions destroyed much of the market value accumulated during the speculative boom of the 1920s.
The crash had been encouraged by margin trading, in which investors borrowed heavily to buy shares. As prices fell, brokers demanded repayment or additional collateral. Forced selling created a feedback loop: falling prices caused more margin calls, which caused still more selling.
The stock market did not reach its ultimate low immediately. The Dow continued falling and eventually bottomed in July 1932, roughly 89% below its 1929 peak. The market crash deepened the economic crisis, but historians also identify banking failures, deflation, unemployment, and international financial problems as major causes of the Great Depression.