IATA describes interline as a relationship in which one airline sells a customer services provided by another, so that airlines can serve markets they cannot serve alone. MITA, IATA Resolution 780, is a single agreement that IATA and non-IATA airlines can join. It covers fares and schedules, accepting each other’s passengers and bags, interline billing and baggage claims. It assumes the use of electronic tickets. Two airlines interline only once both have agreed to deal with each other, and settlement agreements help decide each airline’s share of the revenue.

How it shows up for agents

• One ticket, several airlines. Virgin Atlantic will act as validating carrier only where it has a valid interline agreement with each airline flying the passenger.

• Two-way or one-way. In a bilateral agreement both airlines accept each other’s tickets and check bags through. In a unilateral agreement only one accepts the other’s tickets. Virgin Atlantic lists bilateral agreements with more than 60 airlines, including British Airways.

• Help in disruption. IATA’s framework provides through check-in for passengers and bags. It also provides re-accommodation when flights are disrupted.

Worth knowing

• Separate tickets are different. IATA uses the term virtual interline for itineraries that an intermediary puts together from several airlines, often with no interline relationship between them, and says these may not give the same service or protection. The CAA says passengers with separate bookings have no statutory right to care, compensation or onward travel if a delay makes them miss a later flight, though each flight keeps its own entitlements.

• Codeshares go further. IATA describes codeshares, joint ventures and alliances as closer commercial arrangements, and notes that interline now happens more within them.

• Low-cost carriers. IATA says the complexity of its framework can limit the involvement of low-cost carriers, and that many airlines may not follow its standards because they use a different model, such as ticketless.