Nearly every month, the U.S. Department of Transportation imposes hefty fines on an airline for its failure to disclose that the advertised carrier is not actually operating the flight. This code-sharing, though common, continues to be a source of confusion to travelers.

The DOT describes code-sharing as a marketing arrangement in which one airline puts its code on the flights of another airline to coordinate services and advertise and sell the other airline's services as its own.

Under such carrier agreements, tickets are sold to passengers under a single airline's identifier code, purporting that the passenger's journey will be flown exclusively on that single carrier. In fact, one or more different airlines actually operate flight segments on an itinerary for which the passenger has reserved space. The short of it is that you can purchase a ticket from New York to Helsinki on American Airlines, yet you will fly the nonstop trip on Finnair under an airline code-sharing agreement.

All major United States airlines share codes with other carriers, domestic as well as foreign-flag carriers. The current champ is American Airlines, with 28 domestic and international code-share partners. The airlines sell tickets on each other's flights, either by buying a block or by dipping directly into the other line's seat inventory.

And they can sell the tickets at whatever price they please because the carriers are prohibited from consulting and fixing fares. The practice of code-sharing means that an airline can advertise destinations it does not fly to and, at the least, get a passenger's business for any flights that feed into the partner's flight.

To alleviate the appearance of deception, the DOT enacted regulations in 1999 that were designed to inform the flying public of the actual operator of air flights. These rules require that any advertisement by an air carrier or ticket agent of code-share service must disclose the code-share relationship and the identity of the carrier providing the service. In addition, airline agents and ticket agents must inform prospective passengers at the earliest opportunity during calls to reservation offices when the flight involves a code share and must identify the carrier actually providing the service, even if the customer has not asked to book the flight.

These DOT regulations affect not just code-sharing, but also a practice obscurely known as "change of gauge," an old railway term that refers in airline parlance to what happens when you take a one-stop flight and discover that at the connecting airport, you also have to change planes. These flights are often described as "direct," terminology that may be confused by the public with "nonstop." Far from it. Now, passengers are required to be advised if they must transfer to another plane, even if it is the same aircraft model and bears the same flight number.

GLOBAL DOMINANCE

Code-sharing is the most visible consequence of vast alliances that airlines are forging in their quest for global dominance. These accords include loose marketing agreements, reciprocal sales and marketing pacts, coordination of schedules and fares, cross-equity holdings and aircraft-buying consortia, and franchising deals where one airline pays another for the right to carry its name. Creating a shared code obviously requires negotiation between airlines, but not necessarily much more than that. New names are not painted on airplanes; departure terminals may not be changed; carry-on baggage allowances could be unequal; menu quality may not be similar; and standards of service will vary widely.

Airline mega-partnerships, like United's Star Alliance and American's Oneworld, are promoted by carriers as hugely desirable. But for whom? An "open skies" policy that began in 1978 after deregulation was supposed to mean more competition, more players in the marketplace, more options for travelers — and lower fares.

Critics claim that the only apparent change produced by these code-share alliances may simply be that flights are "relabeled." They point to the negative impact on the flying public: price differential among tickets sold by the code-sharing partners; fewer discount seats available from each airline; reduced choice of flights on some routes; and no consistent rules on baggage allowances or charges, for examples.

A more serious concern is that there are currently no global requirements for aviation safety. Passenger advocates insist that the United States must protect its own citizens by mandating that any foreign air carrier code-share must comply fully with the same high security, safety and competence standards imposed on domestic airliners.

Benefits of code-sharing for the consumer are cited by proponents as shared frequent flier mileage accrual and awards, access to additional lounge clubs, coordinated schedules with "seamless" transfers on worldwide air networks, boarding passes printed out online or at first boarding, and joint ticket sales and reservations.

Evaluating the desirability of your prospective code-shared air itinerary comes down to asking the right questions: