Interline Agreement

An interline agreement is a commercial arrangement between airlines that allows them to issue tickets covering flights on each other's networks, transfer baggage across carriers, and handle connecting passengers as part of a single itinerary — without requiring a full codeshare partnership.

What Is an Interline Agreement?

An interline agreement establishes a set of cooperative rules between two airlines so that a travel agent or airline reservations system can construct a single ticket spanning both carriers. Without an interline agreement, a passenger connecting between two airlines must collect their bags, exit the terminal, check in again, and recheck their luggage — effectively booking two separate journeys. Interlining eliminates that friction by enabling automatic baggage transfer, a single check-in, and a single boarding pass sequence.

Interline agreements predate codesharing and were formalized under IATA's multilateral interline traffic agreements (MITA). Most full-service airlines maintain interline agreements with hundreds of carriers globally.

How It Works in Practice

When a passenger books an interline itinerary, the ticketing airline issues a single document (or set of electronic tickets) that the connecting carrier recognizes as valid for boarding. Baggage is checked through to the final destination, and if the connecting airline causes a delay resulting in a missed connection, it is responsible for rebooking the passenger on the next available service. This protection is what distinguishes an interline ticket from two separately purchased tickets.

Revenue is settled between the carriers through IATA's Billing and Settlement Plan, using standard proration formulas that divide the total fare between carriers based on their portion of the journey. Airlines can also negotiate special prorate agreements (SPAs) for specific markets.

Interline agreements exist on a spectrum. A basic interline covers ticketing and baggage. A special prorate agreement (SPA) adjusts the revenue split. A codeshare goes further by placing one carrier's code on another's flight.

Why It Matters

For passengers connecting between airlines that are not alliance partners or codeshare partners, interline agreements provide meaningful protection — particularly when the itinerary is sold on a single ticket. Airlines value interline agreements because they can offer one-ticket itineraries to destinations beyond their own network, retaining the customer rather than referring them to a competitor.

Key Facts and Figures

• IATA's multilateral interline traffic agreement (MITA) governs standard interline terms.
• Airlines typically maintain interline agreements with 200 to 500 other carriers.
• Revenue proration uses IATA's Rate Construction process to divide fares.
• Low-cost carriers have historically avoided interline agreements but several (including EasyJet and Ryanair) have selectively established them.
• Interline agreements do not require airlines to be in the same alliance.

Related Concepts

Codeshare Agreement, Blocked Space Agreement, Airline Alliance, Special Prorate Agreement, Through Check-in