A codeshare flight is a commercial flight which is operated by one airline, but marketed by others. This concept can be confusing for passengers, but ultimately it can be a valuable service. As a general rule, a flight will be identified as a codeshare flight when tickets are booked, for the convenience of the passenger.

The concept was pioneered in 1990 by an agreement between American Airlines and Qantas, an Australian carrier. The two airlines reached a cooperative agreement which merged many of their flights. Essentially, the agreement allowed American Airlines to sell tickets on Qantas flights, and for Qantas to sell tickets on American flights. The result was that each airline could service more areas, without technically expanding its service.

Multiple airlines began to follow suit, and it is not unusual to see three or four airlines all selling tickets on the same flight. The “code sharing” refers to the idea that each airline generates a separate flight code, such as AA125 for “American Airlines flight 125,” so a flight may be identified by multiple codes for passengers, depending on which airline they booked tickets through.

The airline which actually runs the flight is called the “operating carrier.” This carrier is responsible for all service provided, and when people check in for the flight, they are generally expected to go to the desk of the operating carrier, regardless as to which airline sold the original tickets. Air traffic control identifies the flight by the flight code of the operating carrier, although airport status screens such as arrival and departure boards may list multiple flight codes for the same flight, reflecting the codesharing agreement.

Carriers which sell tickets on other airlines are known as “marketing carriers.” Many passengers prefer to stick with one marketing carrier, so that their frequent flier miles and benefits are consolidated. In many cases, a marketing carrier has multiple codeshare agreements, to ensure that customers will get the flight service that they need.

For airlines, a codeshare agreement is a very useful service, because it allows them to easily network with other airlines. Passengers also tend to find codeshare agreements useful, although they can get frustrating. Through a codeshare, for example, luggage can be easily transferred between multiple airlines on a flight which has more than one leg, and passengers can also keep one frequent flier mile account, rather than multiples. Codeshare service also allows passengers to travel to areas which their preferred carrier does not normally cover.