Codeshare
What is Codeshare?
A codeshare is a commercial arrangement where two or more airlines share the same physical flight, but each sells seats under their own airline code and flight number. The flight is operated by one airline (the "operating carrier"), while the other airline (the "marketing carrier") sells tickets for those same seats under a different flight number.
For passengers, codeshares can be confusing: you book through Airline A, arrive at the airport, and discover you're actually flying on Airline B's aircraft. The airlines involved are typically alliance partners — Star Alliance, oneworld, or SkyTeam. Codeshare tickets often allow you to earn frequent flyer miles on either airline involved and may be combinable with other fares from the marketing carrier.
Example of Codeshare
Sofia booked what appeared to be a straightforward British Airways flight from London to Kuala Lumpur. At check-in, her boarding pass read "Malaysia Airlines MH0001." The staff were Malaysian, the food was Malaysian, the entertainment system was different from what she'd expected — but her Avios points still accrued with BA. She made a note to always check which airline was actually "operating" before assuming she knew what she was flying.