• The New York Times , Thursday, July 31, 2008 12:31 PM

Beginning next month, Delta Air Lines will implement a three-tier system in its frequent flier program. It is reinstating a feature called "last seat," which allows a frequent flier to book any remaining seat on a plane, whether or not it is designated for a member of its mileage program.

Previously, Delta offered frequent-flier tickets for domestic coach travel at either 25,000 or 50,000 miles. Under the new system, travelers will need 25,000, 40,000 or 60,000 miles, depending on when they book their tickets, and where they are traveling. Alaska Airlines announced a similar three-tier program last week that will take effect in November, but other big airlines have not followed.

Jeff Robertson, the managing director of Delta's SkyMiles program, says the number of miles issued by Delta has grown 24% from 2004 to 2007, but the number of seats available on its planes has not increased. "The capacity is just not there," Robertson says. Details are available at http://www.delta.com/skymiles upgrades.

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