In the United States, the market is overwhelmingly dominated by the three legacy carriers, United Airlines, American Airlines, and Delta Air Lines, all of which dominate the air travel market in almost all US cities. However, there are some notable exceptions where non-traditional airlines play a major role and the most successful of these has undeniably been 
Southwest Airlines, which stands out as the only low-cost airline to maintain a market share that is anywhere near challenging that of the three largest carriers.