This is incoherent for a single order. When the first order hits the books, a subsequent order has no opportunity to execute before the first except at a price

This is incoherent for a single order. When the first order hits the books, a subsequent order has no opportunity to execute before the first except at a price disadvantage. Front-running typically involves an agent (i.e. a broker) executing on behalf of the principal, where the agent makes his own trade first, moving the market against the principal, and only then executing the principal's trade. Notice: two orders. This is not a problem when the principal has an exchange seat and may execute directly. Where firms are performing multiple-exchange arbitrage, and the principal splits a large order into several for execution across multiple exchanges, there is now an opportunity for something like front-running: the arbitrage firm can detect the principal's intent from the first order to hit the first exchange, and then try to move the market profitably (against the principal) on the other exchanges before the correlated orders can execute. While somewhat analagous, this activity should not be referred to as front-running.