Originally Posted by
nytango
it is not the one you "booked" with.. The last airline you traveled on is the responsible party.....
That's the key. The last airline involved in carrying you to your destination is responsible for your checked bag, irrespective of the booking (direct, code share, FIM'd due to irrops, whatever). And that's not just a happy agreement that holds between AA and BA: it's the IATA standard.