While passengers may look at a multi-stop trip as a seamless journey involving several planes, airlines must sometimes negotiate interline agreements to accomplish the task.
Each major carrier has its own ticketing system that it uses to keep track of passengers across dozens or hundreds of flights each day. Normally, a ticketing system is not compatible with those of other airlines.
An interline agreement allows passengers to add flight legs to an itinerary from one airline by using a second carrier's flights.
Unlike interline agreements, code sharing agreements principally offer passengers the ability to gain rewards on flights with partner airlines while using another carrier.
Interline agreements increase travel flexibility for passengers, who can book travel using two separate airlines without issues of comparing separate flight schedules.
An airline can use an interline agreement to increase services for customers, and since the contract can be unilateral, an airline can offer flights from other carriers without its flights being made available to the second party's customers.
