Mainline carriers are the main operating unit of an airline and do not include regional alliances and subsidiaries. Legacy carriers operated inter-state routes before the airline industry was deregulated in the 1970s. Many legacy airlines have ceased operations or merged with other carriers. Legacy carriers have hub-based operations and, as of March 2011, these airlines include Delta, American, United, Continental and US Airways. Hawaiian Airlines and Alaska Airlines are also considered legacy airlines, although they do not operate extensive national networks.
In addition to legacy airlines, there are a number of other mainline carriers that operate flights within the United States. Some of these airlines are considered low-cost or no-frills airlines, which offer limited or no amenities, such as in-flight meals and entertainment. Non-legacy mainline airlines include AirTran Airways, Allegiant Air, Frontier Airlines, JetBlue Airways, Southwest Airlines, Spirit Airlines, Sun Country Airlines, USA3000 Airlines and Virgin America.
Generally, regional carriers operate domestic flights on behalf of mainline carriers. Atlantic Southeast Airlines, Chautauqua Airlines, Comair, Compas Airlines, Mesaba Airlines, Pinnacle Airlines, Shuttle America and SkyWest Airlines operate flights for Delta's regional brand, Delta Connection. Others are subsidiaries of mainline airlines. For example, American Eagle and Executive Airlines are subsidiaries of AMR Eagle Holdings Corporation, which is also the parent company of American Airlines. Regional airlines include Air Wisconsin, Cape Air, Colgan Air, CommutAir, Era Aviation, ExpressJet, Go! Mokulele, GoJet, Grant Aviation, Great Lakes Airlines, Gulfstream International, Horizon Air, Island Air, Kenmore Air, Lynx Aviation, Mesa Airlines, Nantucket Airlines, Pacific Wings, PenAir, Piedmont Airlines, PSA Airlines, Republic Airlines, SeaPort Airlines and Trans States Airlines.
Commuter airlines, or air taxis, offer on-demand flights and charters on aircraft generally with 9 seats or less. Often these airlines serve limited routes within a specific area or region. Commuter airlines with domestic services in the United States include Air Choice One, Arctic Circle Air, Bering Air, Everts Air, Frontier Flying Service, Hageland Aviation Services, San Juan Airlines, SeaCoast Airlines, Servant Air, Taquan Air, Vieques Air Link, Warbelow's Air Ventures, Wings of Alaska and Wright Air Service.
