Interline Agreement
A commercial arrangement between two airlines allowing them to handle each other's passengers and baggage on a single booking.
What is an interline agreement?
An interline agreement is a commercial arrangement between two or more airlines that enables them to issue tickets and handle baggage for each other's flights on a single itinerary. If you buy a single ticket routing from London via New York to a small regional US airport, it is likely that an interline agreement covers the connection between a major carrier (say, British Airways) and a regional feeder carrier (say, Cape Air).
Without an interline agreement, a connecting flight booking is just two separate tickets sitting next to each other. With one, the airlines co-operate: your bags are checked through to the final destination, delays on the first flight trigger an automatic rebooking obligation on the second, and a single booking reference covers the whole journey.
Interline vs codeshare
An interline agreement is simpler and broader than a codeshare. Two airlines with interline agreements do not sell seats on each other's flights — they just co-operate operationally when a passenger's itinerary involves both carriers on the same booking. A codeshare goes further, allowing one airline to sell seats on the other airline's operated flights under its own flight number.
Alliance membership typically includes interline agreements with all member airlines as standard. Non-alliance interline agreements exist too — especially between low-cost carriers and regional operators who are not in any alliance.
Why interline matters for travellers
When booking connecting flights, always check whether the airlines involved have an interline agreement if you are buying through a third party. If they do not, and the first flight is late, you are responsible for rebooking yourself on the second at whatever fare is available — which could be expensive. Budget airlines like Ryanair and easyJet have limited interline agreements, which is why combining them with other carriers on multi-leg trips carries risk.
Many booking platforms now show "protected" or "self-transfer" labels to indicate whether a connection is interlined. Choosing a protected connection costs slightly more but provides peace of mind on tight itineraries.