What is an interline agreement?
An interline agreement is a commercial arrangement between two airlines that lets them sell each other's flights on one ticket, check baggage through to the final destination and rebook each other's passengers when something goes wrong. It is the plumbing behind most multi airline itineraries.
Interline agreement explained
Without one, two airlines cannot combine into a single ticket at all. Every multi carrier itinerary you have ever flown on one booking reference exists because an interline agreement sits behind it, setting out how the fare is divided, whose rules apply and what happens when a connection fails.
It is a lower level of cooperation than a codeshare or an alliance. A codeshare puts one airline's flight number on another's aircraft and is marketed jointly. An alliance adds shared lounges, reciprocal status recognition and mileage earning across the group. Interline sits underneath all of that as the minimum required to sell a through ticket.
The passenger benefit shows up when things break. On an interlined ticket a delayed first leg obliges the airlines to sort out the rebooking between them, and your bag is tagged through rather than needing collection at the intermediate airport. Neither happens on two separately bought tickets, however well the times line up.
The absence of an agreement is what forces some itineraries onto separate tickets. Low cost carriers historically held very few interline agreements, which is exactly why routings that combine them with a full service airline appear as self transfer rather than as a protected connection.
More on interline agreement
Interline is the baseline arrangement that allows two airlines to sell a combined ticket and check bags through. A codeshare goes further, placing one airline's flight number on another's aircraft and marketing it jointly.
Usually yes, and that is one of its main practical effects. Some itineraries still require you to collect and re-check bags, most often when a country demands customs clearance at the first point of entry.
Because no interline agreement exists between them. Low cost carriers have historically held very few, which is why routings combining them with full service airlines are sold as self transfer instead.
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