If you are flying on a "codeshare" flight—where the flight is marketed by one airline but operated by another—the rule of thumb is that you must check in with the "operating" carrier. This is the airline that actually owns the airplane and employs the pilots. For example, if you bought a ticket on the American Airlines website but the flight says "Operated by British Airways," you should go to the British Airways desk at the airport or use the British Airways app to check in. This is because the operating carrier handles the seat assignments, baggage logistics, and boarding passes. However, in 2026, many "Joint Business Venture" partners like AA and BA have improved their digital integration; sometimes the AA app will redirect you seamlessly to the BA check-in portal. If you have checked bags, you always drop them off at the counter of the airline that is flying the first leg of your journey. If you are unsure which airline is operating your flight, look at your confirmation email for the phrase "Flight [Number] operated by..."; that is your cue for which check-in counter to visit.