In brokerage offices, employees were kept busy—many of them for most of the night—at various special chores, of which by far the most urgent was sending out margin calls. A margin call is a demand for additional collateral from a customer who has borrowed money from his broker to buy stocks and whose stocks are now worth barely enough to cover the loan. If a customer is unwilling or unable to meet a margin call with more collateral, his broker will sell the margined stock as soon as possible; such sales may depress other stocks further, leading to more margin calls, leading to more stock
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