. . . Kiwi Flyer's take on both the pros and cons of codeshares:

"First, codeshares increase the number of flights and destinations available through an airline. This improves the travel options for us and can help reduce fares, since you can book the codeshare on a single fare in combination with other flights. As savvy travellers know, having an itinerary on the same ticket helps protect connections. On separate tickets, if you miss the connection, you could be on your own. In fact, many airlines offering codeshares improve their schedules to make connections easier--another benefit.

Codeshares can, however, be a trap for the unwary in that you check in with the operating airline, not the airline whose flight you are booked on. For this reason, your itinerary supplied by your travel agent should include the operating airline's name and flight number.

All codeshares earn miles in the frequent-flier program of the airline carrying the code (not just the one operating the flight). Depending on the airlines involved, the mileage may or may not count toward elite status. In general, I find that One World Alliance frequent-flier programs count it toward status, even if the operating airline is not a One World airline, as long as the code is of the frequent-flier program's home airline. In contrast, Star Alliance frequent-flier programs generally do not count the codeshare flight toward status, unless the operating airline is a Star Alliance member.

In general, codeshares make it harder to upgrade, since the operating airline decides the upgrades, whether free or paid for with miles or vouchers. Many upgrade vouchers and miles cannot be used on codeshares. The only ones I am aware of that can be used are in the Star Alliance mileage-upgrade scheme, which is rather more expensive than if you upgrade using miles or vouchers from the operating airline's own frequent-flier program and usually requires a higher paid fare as well.

On a more positive note, seat availability can differ between operating airline and codeshare airline for the same flight. This is because the codeshare airline has in effect reserved some seats and may be faster or slower at selling those seats than the operating airline. Sometimes this means a cheaper fare, or better award availability, on the codeshare, and sometimes it is the other way around." 

In other words, as I've written in past Perrin Reports, when you're considering booking a flight, ask if it's a codeshare. If so, call BOTH airlines to see what the fare is. It could be hundreds of dollars lower on one of the airlines, even though it's for the same seat on the same flight.

Thanks, Kiwi Flyer, for leading us through the codeshare maze.