When you buy a theater ticket, you expect to see a show and not be told, "Sorry, all the seats are taken," when you arrive for the performance. But that's not how our nation's airlines operate.

It's standard procedure for airlines to overbook or sell the same seat twice on any given flight. When two passengers are holding a ticket to the same seat and the flight's full, someone has to give. And that's when bumping occurs.

Believe it or not, bumping is relatively rare. According to the

Department of Transportation

, of the 135 million people who flew during the second quarter of 1999 on the nation's 10 largest airlines, just slightly fewer than 12,000 were involuntarily bumped. "The numbers on bumping are astonishingly low," says Joe Brancatelli, a New York, N.Y. airline consultant and writer. "On the list of problems with airlines -- and that's a big list -- bumping isn't a big problem."

But it can be for somebody who is bumped, particularly business travelers who have to get somewhere and aren't really all that interested in the apologies, free tickets and even cash the airlines offer to ticket-holding passengers who are booted from a plane.

"A very, very small percentage of travelers ever get

involuntarily bumped from a flight," says Chris McGinnis, an Atlanta-based travel writer and the author of

The Unofficial Business Traveler's Pocket Guide. The airlines had a rate of denying boarding of 0.89 per 10,000 passengers during the second quarter of 1999, better than both the 1.44 rate for the first quarter of the year and the 1.01 mark for the second quarter of 1998, according to

TheStreet.com Internet Sector

index. "But if you're bumped, the payoff can be pretty lucrative. You just have to know your rights and be aggressive with the airlines," McGinnis advises.

All major airlines overbook, claiming the procedure benefits passengers and helps keep costs down. "We do this because historical information shows that relatively few people cancel their reservations when they change their travel plans," is how

AMR's

(AMR)

American Airlines

explains overbooking on its Web site. "Overbooking is done in the best interest of both customers and the airline. Without the revenue produced by filling seats that would otherwise go empty, every airline would have to compensate by raising fares," American says.

Voluntary bumping, of course, occurs when you're waiting at the gate or even on the plane and a flight attendant or ticket agent asks if you would like to give up your seat and take the next flight to your destination. Seeking volunteers, by the way, is a federal government requirement.

To entice a volunteer, the airlines start offering goodies. "Usually the inducement is a free round-trip ticket or a dollar-value voucher good for future ticket purchases," McGinnis says. "Agents also are empowered, but not required, to sweeten the pot with other perks such as hotel vouchers, meal vouchers, free prepaid phone cards, passes to the airline's airport club or even cash."

Joel Stark, an American Airlines flight attendant for the past 22 years, likens the process to an auction as ticket agents start offering travel vouchers to passengers on overbooked flights. "They start low, then increase the offer until enough boarded passengers accept the offer," he explains. "Sometimes it goes quite high."

McGinnis says the airlines are happy to pass out these freebies because they don't want to deal with an involuntary bumping situation, one of the few situations for which the government requires specific cash compensation.

Here's how the compensation system works: When you're involuntarily bumped, ask for a seat on the next flight to your destination on that airline or on another airline. Travel experts say the airlines may not always be totally forthcoming about the availability of their competitors' flights, so if you have time, take a walk to another terminal and check the departure schedule. The airline that bumped you pays the full cost of your ticket on the other airline.

If the flight gets you to your destination within an hour of your originally scheduled arrival time, there's no money in it for you. But if you arrive between one and two hours late, you're entitled to a cash payment equal to the price of your one-way fare, up to a maximum of $200. More than two hours, the compensation goes up to a max of $400.

But under federal law, you must have a reservation to be eligible for compensation, and you must check in at the departure gate at least 20 minutes before the flight's departure time.

McGinnis says airlines will often try to appease involuntarily bumped passengers with free tickets or free vouchers. But he warns that if you accept one of these, you're forfeiting your right to the cash. "Sometimes a free ticket or voucher might be more valuable than cash. But if you prefer the cash, stick to your guns and demand it, citing the government rules," he says.

As part of their push to improve service -- a move spurred by a record number of complaints during this past summer's travel season -- the airlines are changing some of their policies regarding overbooking. Both American and

UAL's

(UAL) - Get Report

United Airlines

say that if passengers want to know if a flight is overbooked, they will advise them at the time reservations are made. But they're not likely to volunteer that information, so Brancatelli suggests you ask if the plane is overbooked while you're making your reservation.

Delta Air Lines

(DAL) - Get Report

, which had the worst record for bumping between April and July, also has made changes. Reservation and customer-service agents will inform a passenger if a flight is overbooked. A passenger who gets bumped will receive a ticket on the next available flight -- either on Delta or another airline -- and meal and hotel vouchers if the passenger can't be placed on another flight the same day.

In fairness to the airlines, Brancatelli says frequent travelers -- particularly those who often fly for business -- double book "all the time" to make sure they can get to a destination on a certain day, or because they have uncertain schedules and aren't sure when they'll be able to leave town.

His solutions? No refundable tickets for passengers who book more than one flight. "If you want to triple book to guarantee you can get a certain flight, you pay for it. And, for the airlines, no overbooking. They sell a seat to you, you're in it or it's empty."

Patrick Crowley is a political reporter and columnist for The Cincinnati Enquirer. At time of publication, he held no position in any securities mentioned, although holdings can change at any time. Crowley can be reached at

crowleys@cinci.infi.net.