Four giant airline computer systems in the United States handle nearly all the airline reservations in the country. (They're known as CRSs, for computer reservations systems, or more often now GDS for global distribution systems.) Although each airline has a "home'' CRS, the systems are all interlinked so that you can, with few exceptions, buy tickets for any airline from any CRS. The dominant systems in the U.S. are Sabre (home to American and US Airways), Galileo (home to United), Worldspan (home to Delta, Northwest), and Amadeus (Continental and many European lines.) Many of the low-price start-up airlines don't participate in any of these systems but have their own Web sites where you can check flights and buy tickets. Southwest, the largest and oldest of the low-price airlines, doesn't participate, either. Southwest's web site gets car and hotel info from Galileo, but the info seems not to flow the other way. Orbitz, one of the big three online travel agencies, runs its own system which is linked directly to many of the airlines.
In theory, all the systems show the same data; in practice, however, they get a little out of sync with each other. If you're looking for seats on a sold-out flight, an airline's home system is most likely to have that last, elusive seat. If you're looking for the lowest fare to somewhere, check all four systems because a fare that's marked as sold out on one system often mysteriously reappears on another system. Some airlines have rules about flight segments that are not supposed to be sold together even though they're all available, and at least once I got a cheap US Airways ticket on Expedia, which didn't know about all the US Airways rules even though I couldn't on their own site or Travelocity which did know about them. On the other hand, many airlines have available some special deals that are only on their own Web sites and maybe a few of the online agencies. Confused? You should be. We are.
The confusion is even worse if you want to fly internationally. Official fares to most countries are set via a treaty organization called the IATA, so most computer systems list only IATA fares for international flights. It's easy to find entirely legal "consolidator'' tickets sold for considerably less than the official price, however, so an online or offline agent is extremely useful for getting the best price. The airlines also can have some impressive online offers on their web sites. http://airinfo.aero/
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