Many travelers have become their own travel agents in recent years, thanks to the Internet's ascendancy. But judging from the almost daily calls and e-mails to The Seattle Times Travel department, many travelers are frustrated and feel like they aren't finding the cheapest airfares.

They might not be finding the best deals. Airfares are a constantly moving target, due to airlines' computerized "yield-management" systems. Fares can change daily, sometimes hourly, depending upon how quickly seats are selling on a given flight. The higher the demand for seats, the higher the fare. And there can be more than a dozen fares on a single flight.

This summer, it could be hard to find cheap tickets for both domestic and international plane trips.

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