All six instituted a policy that nonrefundable tickets are worthless if not used by the day specified on the ticket. If you miss your flight , you lose out unless you meet the airlines' "flat-tire" policies, outlined below.

U.S. Airways' rule went into immediate effect, while the other five airlines' apply it to travel  purchased on or after Aug. 30 for travel on or after Oct. 1.

All six still allow passengers with nonrefundable tickets to change their itinerary by calling ahead, but travelers  must choose their new flight on the spot and pay any price difference on the new fare  then. The $100 change fee on domestic flights still applies for all six of the airlines. That charge varies by airline on international flights.

Here are more details on the rule changes, by carrier. Warning: These rules can, and sometimes do, change at a moment's notice.

American Airlines (AMR: news, chart, profile)

• If you miss your flight on a nonrefundable ticket, you can fly on a standby flight without charge if you get to the airport within two hours of your flight's scheduled departure.
• Stand-by fees on nonrefundable tickets will be $100 as of Jan. 1.
• Fee for changing itinerary on a nonrefundable ticket for international travel now costs $100 to $200.
• Passengers are allowed two checked bags. Excess bags cost $80 each, as do bags that are overweight  (more than 70 pounds) or oversize (more than 62 inches when height, width and length are added together). These charges are cumulative -- a third, overweight, oversized bag will run you $240.
• Paper ticket charge: $20.
• Meals are no longer offered in coach class on flights that are less than four hours. Alcoholic drinks are still free on most international flights.
• For unaccompanied minors there is a mandatory charge, each way, of $40 on nonstop flights and $75 on flights with connections.
• Bringing a pet costs $75.

Continental Airlines (CAL: news, chart, profile)

• If you miss your flight on a nonrefundable ticket, you can fly on standby without charge if you get to the airport within two hours of your flight's scheduled departure.
• Stand-by fees on nonrefundable tickets will be $100 as of January 1, 2003.
• Fee for changing the itinerary on a nonrefundable ticket for an international flight: $100.
• Two checked bags are free. Excess, overweight or oversize bags cost $80, and the fees are cumulative.
• Paper ticket charge: $20.
• Meal service is unchanged. Alcoholic drinks on international flights are no longer free.
• Unaccompanied minors: $40 on one-way nonstop flights, and $75 on connecting flights.
• Pets: $80.

Delta Airlines (DAL: news, chart, profile)

• If you miss your flight on a nonrefundable ticket, you can fly on a standby flight -- for a fee of $100 -- if you get to the airport within two hours of your flight's scheduled departure.
• Stand-by fees on nonrefundable tickets will be $100 as of January 1, 2003, except tickets for Delta Express flights will be charged $50.
• Fee for changing the itinerary on a nonrefundable ticket for an international flight: $150 to $200.
• Two checked bags are free. Excess, overweight or oversize bags cost $40, and the fees are cumulative.
• Paper ticket charge: $20.
• Meals have been eliminated in coach class on flights fewer than 1,750 miles. There is no charge for alcoholic drinks on international flights.

Northwest Airlines (NWAC: news, chart, profile)

• If you miss your flight on a nonrefundable ticket, you can fly on a standby flight without charge if you get to the airport on the day of your flight's scheduled departure. BizFlex fares are exempt from the new rule.
• There is no fee for flying stand-by.
• Fee for changing the itinerary on a nonrefundable ticket for an international flight: $200.
• Two checked bags are free. Excess, overweight or oversize bags cost $80, and the fees are cumulative.
• Paper ticket charge: $25.
• Meal service has been discontinued on some shorter flights. Alcoholic drinks on international flights continue to be free.
• Unaccompanied minors: $40 on one-way nonstop flights, and $75 on connecting flights.
• Pets: $80 in cabin.

United Airlines (UAL: news, chart, profile)

• If you miss your flight on a nonrefundable ticket, you can fly on a standby flight -- for a $100 change fee -- if you get to the airport on the day of your flight's scheduled departure.
• Stand-by fees on nonrefundable tickets will be $100 as of January 1, 2003.
• Fee for changing the itinerary on a nonrefundable ticket for an international flight: $150 to $200.
• Two checked bags are free. Excess, overweight or oversize bags cost $75, and the fees are cumulative.
• Paper ticket charge: $20.
• Hot meals, in general, are not available on flights shorter than two-and-a-half hours. Alcoholic drinks on international flights continue to be free.
• Unaccompanied minors: $60 on all flights.
• Pets: $75 in cabin.

U.S. Airways

• If you miss your flight on a nonrefundable ticket, you can fly on a standby flight at no charge if you get to the airport within two hours of your flight's scheduled departure.
• Stand-by fees on nonrefundable tickets will be $100 as of January 1, 2003.
• Fee for changing the itinerary on a nonrefundable ticket for an international flight: $100 to $200.
• Two checked bags are free. Excess, overweight or oversize bags cost $80, and the fees are cumulative.
• Paper ticket charge: $25.
• Meal service has been reduced on flights less than 1,000 miles.
• Pets: $100.