1. Airlines provide regularly scheduled service between a city of origin and a city of destination. Such origin-destination combinations are known in the industry as "city pairs." Airlines may offer city-pair service on a "nonstop" basis or on a "connecting" or "one-stop" basis that requires a passenger to make one or more stops en route and perhaps change planes on the way. Passengers traveling on a particular city-pair route do not view service in alternative city pairs as a reasonable substitute: they are unlikely to substitute travel to a different destination in response to a fare increase for the city-pair service they desire. Unless travelers' destination cities are located close to their origin cities, few will regard other modes of transportation (e.g., automobile, bus, or train) as reasonable substitutes. Airline passenger service in a city pair constitutes a relevant market for antitrust purposes.