The root cause of Thursday's Nasdaq Stock Market trading halt can be traced to a single word: complexity.

Trading in stocks today takes place on dozens of trading venues: 13 stock exchanges; some 40 dark pools, or private trading platforms; and countless "internalizers," or trading operations in which stockbrokers match trades inside their own walls. Increasingly, all this trading takes place at speeds measured in the millionths—or billionths—of a second inside giant data centers housing state-of-the-art supercomputers.

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