Every evening, after a futures exchange such as the Merc closes, the process of clearing is undertaken. Those whose trading positions have lost money must transfer cash or collateral to the exchange's clearinghouse for deposit into the accounts of those whose positions have gained. After a normal day on the Merc in the late 1980s, $120 million would change hands. On the evening of October 19, however, those who had bought S&P futures contracts owed those who had sold such contracts twenty times that amount (Melamed and Tamarkin 1996, p. 359).

Donald MacKenzie, An Engine, Not a Camera, loc. 19