"On the other hand it does feel a little weird to protect quotes that you can't instantly trade against. If a market delayed orders by 10 minutes, instead of 350 microseconds, it would be sort of unfair to force investors to trade there rather than take an apparently inferior but immediately available price elsewhere. And this is something that the SEC has actually thought about. The order protection rule is Rule 611 of Regulation NMS, which requires a trading center "to prevent trade-throughs on that trading center of protected quotations." Those terms are defined in Rule 600; the meaning for our purpose is that an exchange can't execute a trade at a price lower than the best bid (or higher than the best offer) of any displayed "automated quotation" on another exchange. An "automated quotation" is one displayed on an exchange that "immediately and automatically" responds to incoming orders. And the SEC, in adopting Regulation NMS, had this to say about that requirement:"
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