Here's a look at what the proposed merger of American Airlines and US Airways Group Inc. would mean for competitors. They're listed in order by size:
UNITED CONTINENTAL HOLDINGS INC.
"The merged American will be a formidable competitor," Chairman and CEO Jeff Smisek told workers in a message on Thursday. The combined airline will have a similar route network — for instance both fly from New York to Chicago — though United's is stronger, he said. Smisek also noted that American will exit bankruptcy court with a "significantly lower cost structure" than United.
DELTA AIR LINES INC.
Along with United, Delta will be the other big international competitor to a merged American. Delta itself is the product of a 2008 acquisition with Northwest. CEO Richard Anderson said in December that "having concluded (Delta's) mergers quite successfully has given us a significant lead in the industry and it's a lead that we don't intend on relinquishing."
SOUTHWEST AIRLINES CO.
Southwest has aggressively added St. Louis flights as American reduced service. The new American is widely expected to cut flights in Phoenix, too, which could allow Southwest to do the same thing there.
JETBUE AIRWAYS CORP.
New York-based JetBlue has a close relationship with American, and both airlines have big operations at John F. Kennedy International. CEO Dave Barger told CNBC on Thursday that most mergers result in lower airline capacity — giving JetBlue a chance to fill those gaps.
Alaska has strong links to Delta, which helps feed traffic into Alaska's Seattle hub. Alaska's core territory is for flights up and down the West Coast, and even the merged American will still trail Delta and United in that part of the country.
SPIRIT AIRLINES INC.
The low-cost airline will now face stronger competition on its lucrative Caribbean routes. However, it is aggressively expanding into large cities like Houston, Orlando, Detroit, Denver and Dallas. If American, Delta, Southwest and United raise fares in those cities, more passengers might flock to Spirit.
The airline has a heavy debt load and loses money. It is struggling to lure business travelers, and this merger will probably make that task even harder. However, more leisure travelers who are looking for an alternative flying experience might be attracted to Virgin.
ALLEGIANT TRAVEL CO. and FRONTIER AIRLINES
These two airlines have focused on smaller cities that have trouble attracting larger airlines. If routes now served by American and US Airways are dropped, these two carriers could take advantage of the vacuum.