I'm pretty confused about this, despite being an experienced traveler. I'm attempting to book multi city flights with up to two connections for a trip to Europe from the USA. Because the flights are operated by different carriers, I can't really determine if I would really have to pay for checked baggage, say on the flight from my smaller city to the bigger city in the USA. I've read that transatlantic flights include checked baggage while the booking websites indicate "free checked baggage on flight # 1. There is nothing to click to find the details.
For example, I see this on American's baggage policy: "In some cases, you won't have to pay the fees for up to 3 bags when traveling on itineraries marketed and operated by American Airlines or marketed by American and operated by a oneworld® partner."
So, how do I know I'm an "in some cases" person? Any guidance on this? The flights were are looking at are run by British Airways and their partners.