Matt4 thanks for your description of Marketing and Operating airlines. However this is only half the story. It is the consequences of these definitions that can cause problems.
These definitions were created by IATA to provide 'simple understandable rules for code share flights'.
It does however lead to some interesting anomalies.
Take baggage allowance for example.
Taking the soon to finish codeshare DOH-HKG CX640/QR5801 Your baggage allowance varies according to the marketing carrier.
On CX the standard Y baggage allowance is 20 kgs.
On QR the standard Y baggage allowance is 30 kgs,
If your ticket says CX640 then it is 20 kgs. But if is says QR5801 or 'CX640 sold as QR5801' then it is 30 kgs.
For flights from Europe, The Middle East or Africa the rules are identical since they are all in IATA tariff area 2 whilst HKG is in IATA tariff area 3.
Thus for QR068 FRA-DOH-HKG as well as codeshare AA7968 FRA-DOH-HKG it is the DOH-HKG flight that determines your baggage allowance for the entire journey.
More complicated is
JFK-DOH-HKG
If your flight is QR702 then QRs baggage rules apply throughout your journey. If it is AA7972 then AAs baggage rules apply throughout your journey. The rules for DOH-HKG are ignored.
This is because JFK is in IATA tariff area 1.
It is the change of IATA tariff zone that determines the MSC (Most significant Carrier). For codeshare Carrier means Marketing airline.
These rules can be seen at
http://www.cathaypacific.com/cx/en_H...ier-rules.html and on the IATA website.
Some 18+ months ago I think I saw the comment on the IATA website to the effect that 'With certain exceptions the terms and conditions of the MSC are valid for the entire journey' Am I remembering correctly or incorrectly? If you know definitely one way or the other please let me know.