interline

An interline agreement is a commercial contract that lets one airline issue a ticket for travel that includes flights operated by another airline. The passenger books through a single airline, receives one ticket or itinerary, and the airlines settle payment between themselves afterward. This is distinct from a code-share, where airlines use the same flight number for the same physical flight; interline passengers travel on actual separate flights operated by different carriers.

The mechanics depend on whether the ticket is through or local. A through interline ticket covers the entire journey from origin to final destination, with the selling airline assuming liability for the whole trip, including delays or misconnections on the partner airline. A local ticket covers only one airline's segment. Most interline traffic today runs on prorate agreements, where revenue is split between carriers according to a formula based on distance flown, usually calculated per statute mile or through IATA prorate tables.

Interline agreements also govern baggage handling across carriers. Bags should be checked through to the final destination without the passenger collecting them at the connection, a process called interlining baggage. This requires compatible tracking systems and ground handling coordination; failures here are common pain points when flights are delayed or misconnections occur. Some carriers restrict interline baggage to certain partners or charge fees if the connection time is tight.

The term covers not just passenger revenue but also freight, mail, and charter movements. A shipment might be handed off to a partner airline to complete the delivery, with the originating carrier responsible for arranging and billing. In cargo, interline arrangements are often critical because few carriers serve all airport pairs regularly.

Interline revenue has declined as airlines have built larger networks and consolidated through mergers. Code-shares and airline alliances (like Star Alliance or Oneworld) now handle much of what interline once covered. However, interline remains essential for smaller carriers, regional connections, and routes where a single airline cannot offer convenient schedules. It also provides a fallback when technical failures, strikes, or schedule changes force one airline to use another's aircraft or seats mid-journey.