The concept of fortress hubs is straightforward. While definitions vary, it is where one airline has at least 70% of an airport's flights , which may or may not be achieved by deploying many smaller-gauge regional jets. Of course, dominance will rise further when a carrier's alliance members and partners (interline, codeshare, joint venture, etc.) are added.

While many hubs worldwide meet the 70%+ criterion, those in the US are particularly well-known. A fortress hub is all about control. It is where an airline is so dominant that competition is challenging, due to few spare gates, slots, or otherwise. This power keeps fares to/from the hub higher than they might otherwise be for local passengers , partly offset by the convenience of a vast network and usually high frequencies

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