Interlining (also known as "interline ticketing") is a voluntary commercial agreement between individual airlines to handle passengers traveling on itineraries that require multiple airlines. Some airlines do not have interlining agreements with other airlines or no airlines at all. Interline agreements are usually specific to a reservation or Global Distribution System (GDS) and may allow paper tickets and/or electronic tickets. If an electronic ticket interline agreement exists then the agreement always covers paper tickets also.
Interline agreements are directional. For example it may be possible for American to be the plating carrier on an American-United itinerary but United might not be able to be the plating carrier on the same itinerary.
Airlines which have been established for many years are sometimes called "legacy", "network", or "full service airlines." Previously, only large network carriers such as United Airlines and Lufthansa would have electronic ticket interline agreements but the recent IATA mandate to eliminate paper tickets by the end of 2007 has changed this by forcing smaller carriers to implement electronic ticketing. For example, United Airlines has an interline ticketing agreement with competitors American Airlines and British Airways. Smaller legacy carriers commonly have interline agreements with large network carriers that fly into their markets. Most newer low-cost carrier airlines that only sell directly to consumers (and not through agencies or GDS systems) do not support interlining at all. For example, United Airlines does not have an interline agreement with Southwest Airlines or Easyjet.
If no interline ticketing agreement exists (such as with Southwest and United Airlines) then two separate tickets will need to be issued and passengers will have to retrieve their bags and carry it to the connecting airline for check-in. Itineraries with interline connections such as this (whether issued on a single ticket or two tickets) are risky for travelers since the second carrier may be unaware of delays or issues with the incoming flight and it is more likely for luggage to be lost.
Most online travel agencies will only display itineraries that can be ticketed on one of their booking systems. However Orbitz.com will sometimes display un-ticketable interline itineraries. Examples can be found on routes to Mexico involving Aero California or routes to Indonesia involving Lion Air. These display on Orbitz as "contact airline to buy".
The default behavior for GDS systems is to use the first carrier as the plating carrier. However some agencies may choose a different carrier in the itinerary to be the plating carrier for the convenience of the traveler or to maximize commission.
When changes need to be made to a nonrefundable interline ticket, only the plating carrier must be included in the new itinerary. Other carriers may be added or dropped as needed to build the new itinerary.