Bats’s computers inadvertently permitted trades counter to specific rules. One regulation involves the best nationally available bid and offer, a central concept aimed at ensuring that even as exchanges proliferate, investors who want to buy and sell shares can be confident they are getting the best prices.
The short-sale circuit breaker, implemented in 2011, was intended to prevent investors from driving down share prices when a stock had already fallen 10 percent from the previous day’s closing level. The curb limits the price of short sales relative to the