Code-sharing remains a source of confusion and irritation for travelers. We're talking about the common marketing practice of listing one airline's two-letter code for a flight (such as DL for Delta and AA for American) and having another airline, or even a train, operate the flight. 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, for example, 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 champ is American Airlines with 25 domestic and international code-share partners. It's difficult nowadays to elicit a carrier denunciation of code-sharing, although fewer than 15 years ago, some of them considered it deceptive — paying for one airline's polished, on-time service while receiving another's infamously cramped seating and grumpy attendants.

This switching of airline operators happens every day on hundreds of flights on scores of carriers worldwide. 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 U.S. Department of Transportation enacted regulations in July 1999 that were designed to inform the flying public of the actual operator of air flights. Today, listings in airline guides and on computer screens must show either the code of the airline operator and its flight number, or the code and flight number of the sharing airline, plus an asterisk or other mark to indicate its actual status.

The DOT regulations affect not just code-sharing, but also a practice obscurely known as "change of gauge," an old railway term which 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 as "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.

According to the 1999 DOT rule, travel agents and airlines must offer full disclosure about code-share flights before closing the ticket sale. Written notice with the ticket is also mandated, although the rule does not require that the ticket itself indicate this information. Part of the reason for this omission at the time was the advent of the electronic ticket, basically just a receipt for payment with an air itinerary plus a reservation record locator (aka confirmation number). E-tickets now reign in 2005, with paper tickets costing a fee if specifically requested.

Code-sharing is the most visible consequence of vast alliances that airlines are forging in their quest for global dominance. 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, for examples.

A more serious concern is that there are now 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 flight 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 flyer mileage accrual and awards, access to additional lounge clubs, coordinated schedules with "seamless" transfers on worldwide air networks, boarding passes printed out at first boarding, and joint ticket sales and reservations.

Evaluating the desirability of your potential code-shared air itinerary comes down to asking the right questions: Which airline will be operating this flight? How many times does it stop and where? Is there a change of aircraft or airline? Will class of service, seating and special meal requests be honored throughout the journey, especially if you are transferred to a smaller plane? Is your connection between code-share flights "guaranteed," so that in the case of delays you could expect to be accommodated with food and lodging? How long will you be on the ground? Are you permitted to use an executive lounge? Will you earn frequent-flier miles on either code-sharing airlines? Is there an alternative noncode-sharing route with a cheaper fare?

The bottom line is that airlines have formed huge global alliances to reduce their costs and to increase their sales. But unless market forces increase competition and the cost of fuel decreases, there will be little sign of lower fares, more cabin comfort or improved service.

Irene Croft Jr. of Kailua, Kona, is a travel writer and 40-year veteran globetrotter. Her column is published in this section every other week.