Airport hubs and their evolution

US hub airports are often host to extraordinary dominance wielded by a single carrier or alliance. In some cases, this concentration far exceeds anything seen in other geographies, even where "national" carriers exist. The US is also the only developed aviation market with a multiple hub system. Most other countries have have one primary international entry point at which the dominant carrier focuses its operations. The only exception is Germany, where Lufthansa operates dual hubs at Frankfurt and Munich. In Spain, Iberia is solely focused on Madrid, having long ago discontinued its transatlantic non-stops from Barcelona. Those that remain are now operated by Continental, Delta and American via their New York hubs.