United is the first of the major "legacy" carriers to begin charging for a second bag, noted airline expert Terry Trippler. "Others will follow suit," he said.

The new United policy applies to customers who buy nonrefundable domestic economy tickets. The change took effect with tickets purchased Monday for travel beginning May 5. United will keep its 50-pound-per-bag weight limit for most passengers.

United's most-frequent fliers will be exempt from the rule. United Mileage Plus frequent-flier members with premier status or higher, or silver status or higher with the Star Alliance, will still be able to check two bags for free.

Frontier is considering moving to a "branded fare" or "fare family" type of system, where different fare levels get passengers different amenities. "There's the potential that checked bags or a cost associated with checked bags could become a part of the fare-family structure" at Frontier, Hodas said.

Trippler predicted that by the 2008 holiday season, "everyone will be charging for a second checked bag."

United estimates the change will net it more than $100 million annually. One in every four of its fliers checks a second bag.

Dear United,

Cry me a freaking river. Flying is a big enough pain in the butt as it is. Now you want to gouge me for even more money to fly your lame airline which has cut out perk after perk after perk? Never mind the fact that you pad your flying times by 20-40 min so that you can claim to be the number one airline for on-time arrivals and departures. What a load of crap. Wherever possible, I will continue to fly alternative airlines if it means I can avoid a trip on United. I'll spend my $25 buying the bottled water past security check points since I can no longer bring along my own.

Hugs and Kisses,

LD