4-5 different airlines are involved on a potential itinerary.

Leg 1:

• ticketed by an airline that allows only 1 bag (LH)
• flight number is of that same "ticketing" airline (LH)
• "operated by" an airline that allows 2 bags (SN)

To complicate things, the "operating airline" only allows 1 bag on short flights (and the first leg is indeed short). However, the rest of the itinerary is intercontinental. But the rest of the journey does not involve the generous operating airline of the first leg.

Leg 2:

• ticketed by an airline that allows only 1 bag (LH)
• flight number is of that same "ticketing" airline (LH)
• "operated by" an airline that allows only 1 bag (AC)

Leg 3:

• ticketed by an airline that allows only 1 bag (AC)
• flight number is of that same "ticketing" airline (AC)
• "operated by" an airline that allows only 1 bag (AC)

All return legs:

• ticketed by an airline that allows only 1 bag (UN)
• flight number is of that same "ticketing" airline (UN)
• "operated by" an airline that allows only 1 bag (UN)

According to the documentation for SN, the "operating airline" is the one that sets the rules:
This is quite favorable, if it's true, but almost hard to believe. I've also read that only the first leg matters for setting the baggage rules. However, what about the return journey? Does the first leg on the itinerary set the rules for the return journey as well? Or will I have to lose a bag along the way?

Certainly for the way back it sounds as though it will be one bag, first leg of return journey sets the benchmark. I'm not sure when you have an operating airline with different ticketing, I think it is the metal (when I fly UA / AC codeshare on UA, UA's rules apply and vice versa) even when I am ticketed by AC (although who knows when the rules are more favourable to pax rather than less!)

In my experience, the first airline you check in with will set the baggage allowance for the entire direction of your trip, and the allowance is based on the final destination point.

So in this case that would be SN and since you are flying long-haul, you should be entitled to 2 pieces free of charge on the outbound.

I am a little confused as to how many tickets you have though, because you mention flights 1 & 2 are on LH ticket stock, flight 3 is on AC ticket stock, and all the return flights are on UN ticket stock.

If the first 2 flights are on 1 ticket and the 3rd flight is on another ticket, then it will depend on SN's interline policy if they will be willing to check your luggage all the way to your final destination on the outbound journey. If they won't do that, then after your 2nd flight you will need to re-check your luggage and then you will now be subject to AC's limit of 1 free.

Return journey will be 1 piece based on your info.

This is my experience as well..

The first airline leg.. rules, regulations and charges.. determines what is implemented..

For IATA carriers the "most significant carrier" rule applies for connections, in your example the MSC is AC on the TATL leg. MSC went into effect a few months ago (April 2011), so not many pax have experience with it, and it is quite complex, so those interested can google the full rules -- rarely is the first leg the MSC, though it can be. Also US DOT has separate (more favourable) rules, which only apply for flights on US flag carriers or if O/D is in the USA (neither of which is the case for the OP).

This is my experience as well. Last year I flew a UA/ET (Ethiopian) ticket to Kenya. Even though I checked in with UA first, ET's more generous rules prevailed.

When you say "the metal", is that necessarily the same as the "operating airline"?

And to be clear, does "operating airline" always have the same meaning? E.g., if ITA software says the operating airline is X, is that also true from the standpoint of interpreting the baggage rules? Or is it possible that ITA software says the operating airline is "SN", but SN considers the "operator" to be the airline with the flight number?

What is "pax"? I see that term used a lot.

SN actually replied to an email when asked who's rules will apply, and they said "we have the same baggage policy as LH, so it doesn't matter". Their answer seems bogus to me, because I can see a 1-bag difference when comparing overseas rules on both websites.

To be exact I don't have any tickets.. this is based on ITA software results. It's all given with one price, and ITA software is not capable of multiple bookings, so AFAIK it's 1 booking and therefore 1 ticket.

When you say "LH ticket stock", does that mean LH flight numbers? Is that also synonymous with who the carrier is? Sorry, I'm quite behind on all the terminology that goes on.

I actually have some choices here, considering all the codeshares. If I can assume that "the carrier" = "flight number issuer", these are the choices I have for the first direction:

flight 1 (short):

• operator: SN
• carriers: BD, SN, AC, or LH (but SN turns out to be much more expensive as a carrier, due to an extra "SN YQ Surcharge" - the other carriers are equally priced)

flight 2 (long-haul):

• operator: AC
• carriers: AC, LH, or BD

flight 3 (short):

• operator: QK
• carrier: AC

That's for the outbound journey. I'll leave out the return journey at this point since that question is sufficiently answered.

To help clarify some terms used:
Pax=passengers
Metal=plane
Carrier=airline
Ticket stock=refers to which airline issued the ticket. Their 3-digit number will appear at the beginning of the ticket number. This can be different from the. . .
operating airline=the airline flying the flight. This usually means that it's their crew and metal. This is the airline you would need to check in with when arriving at the airport.

Hope that helps!

Wow that really complicates things! The lawyers must be loving the MSC concept!

Does "TATL" mean transatlantic?

I did some looking, and I can see that ANA (Japan Airlines, which are in the Star Alliance) mentions MSC, but Brussels Airlines does not. I also see that MSC does not apply to the US (ANA says flights with a departure or destination in the US simply use the first leg).

If I consider what you're saying, and also the fact that SN's website states that the policy of the operator is used, does this mean the MSC is a factor of who the operators are? Is it the operator who goes the most cummulative or consecutive miles? Or the operator with the longest single flight? This almost contradicts this thread where someone says MSC comes into play when multiple "carriers" are involved.

If everyone is correct, this would mean multiple carriers triggers the MSC concept, which then is based on who the operator is. But by the name "most significant carrier", it would seem not to matter who the operators are.

Although I suppose it doesn't matter in my case since the US is my destination, and US DOT Order 2009-9-20 seems to negate using MSC for baggage policy, correct? But I'm now considering choosing an itinerary that uses SN as much as possible and then mixes the other airlines as evenly as possible, for good measure. SN is the only airline with favorable rules.

BTW, what does "OID" mean?

And when you say "US flag carriers", do you mean airlines headquartered in the US?

I don't normally check in with the operator. The flight is operated by SN, but in the past I think I have always checked in with the airline who issued the flight number. E.g. if flight LH123 is operated by SN, I will normally tend to check-in with Lufthansa. Are you saying I can check in with SN in this case? I would much rather prefer to do that, because it will likely be harder to convince LH to follow SN rules.

I normally check in (in person or online) with the airline operating the flight, that is, the one whose plane it is, and whose crew it is. I believe that is the norm. So yes, you should absolutely be able to check in with SN.

That's interesting that you've been able to check in with the ticketing airline and not the operating airline. In my experience, I've always been instructed by the ticketing airline to check in with the operating airline, since it's the operator's plane that I'm getting on.

And to answer some of your other definition questions- TATL=Transatlantic flight (TPAC would be a Transpacific flight). US flag carrier would be an airline headquartered in the US (ie United, American, etc). A British flag carrier would be an airline headquartered in the UK (like British Airways). The country that an airline is headquartered affects everything from taxes to where they can fly to what laws they must operate under. "Flag carrier" was originally a nautical term as it applied to ships which often flew the flag of their home countries and carried it to other points of the world.

My memory might be failing me.

It was only recently that I began using ITA software (which indicates who the operator is). Previously I'm not sure how I would know where to go, other than the flight number, or the airline on the receipt. Maybe I've not bought many codeshare flights.

If you haven't bought a codeshare flight before, then the ticketing airline and the operating airline are one and the same, and thus there's no confusion. I fly codeshare flights from time to time and the ticket always states to check in with the operating carrier. As an example, I'm flying a codeshare next month. I have a ticket that was issued by UA and is on UA ticket stock, but all the flights are operated by CO. So when I go to the airport, I will be checking in with CO and getting on a CO plane.

My understanding is that the MSC is usually whichever operating airline is flying the longest segment of the ticket.

You seem to be getting a little confused here. A ticket can only be issued by one carrier, but it can have several flights operated by several different carriers.

If there's just one operating carrier for all the flights, then the operator's rules prevail. If there's multiple operators, then it's the MSC whose rules apply to all the flights (to the best of my understanding)

I'm not sure how that provision might affect the baggage rules.