The New York Stock Exchange traded about 16.4 million shares on 29 October 1929, the Wall Street Crash day called Black Tuesday.
That volume was extraordinary for the period and reflected a rush by investors to sell shares. Prices had already fallen sharply on Black Thursday, 24 October, and again on Black Monday, 28 October. The selling culminated in another severe decline on 29 October.
The crash followed years of rising share prices, widespread speculation, and buying stocks with borrowed money. Although the collapse damaged confidence and contributed to the economic downturn that became the Great Depression, it was not by itself the sole cause of the Depression. Bank failures, falling demand, debt, and policy choices also mattered.
Black Tuesday is often treated as the symbolic climax of the 1929 crash. The number is a trading-volume figure, not the percentage loss of a particular index; those are separate measurements of the event.