This can often cause confusion and even anger, if passengers find themselves on an airline that they consider to be inferior to the one they had chosen. Let’s explain how this circumstance can come about.

Code sharing originated in 1990 when Qantas Airways of Australia and American Airlines combined their services, between a number of US and Australian cities. This preceded the formation of airline alliances, which also use code sharing in a very comprehensive way.

The first alliance, Star Alliance was started in 1997. Thai Airways International was one of the five founding members, and the only one from Asia. This was followed by Oneworld which included Qantas and American Airways and Skyteam led by KLM-Air France.

Most airlines now also have code sharing agreements with other operators. The term ‘code’ is the identifier used in the flight schedule; generally the two character International Air Transport Association (IATA) airline designator code and a flight number.

For example, Flight XX123 operated by the airline XX, might also be sold by airline YY as YY456, and by airline ZZ as ZZ789. Airlines may participate in a code sharing arrangement for several reasons:

1. Connecting flights. This provides clearer routing for the customer allowing a customer to book travel from point A to C through point B under one carrier's code instead of a booking from point A to B under one code and from point B to C under another.
2. Flights from two airlines that fly the same route. This gives an apparent increase in the frequency of service on the route for both airlines.
3. Perceived service to unserved markets. This allows carriers who do not operate their own aircraft on a given route to gain exposure in the market through display of their flight numbers. Under a code sharing agreement, the airline that actually operates the flight (the one providing the ’plane, the crew and ground handling services) is called the operating carrier and the airlines that sell tickets for that flight but do not actually operate it are called marketing carriers.

Most passengers and travel agents prefer flights which provide a direct connection and code sharing gives this impression. It can, however, be annoying for passengers as the Global Distribution Systems (Sabre, Amadeus, Galileo and Worldspan) often do not differentiate between direct flights and code sharing flights.

By Alastair Carthew, a Phuket based writer and communications advisor. Email: alastaircarthew@gmail.com