Why did stock prices drop so quickly in 1929?

The Wall Street Crash of 1929, also known as Black Tuesday (October 29), the Great Crash, or the Stock Market Crash of 1929, began on October 24, 1929 ("Black Thursday"), and was the most devastating stock market crash in the history of the United States, when taking into consideration the full extent and duration of its after effects. The crash, which followed the London Stock Exchange's crash of September, signaled the beginning of the 12-year Great Depression that affected all Western industrialized countries.

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