Interlining, also known as interline ticketing and interline booking, is a voluntary commercial agreement between individual airlines to handle passengers traveling on itineraries that require multiple flights on multiple airlines. Such agreements allow passengers to change from one flight on one airline to another flight on another airline without having to gather their bags or check-in again. Airlines can also promise free rebooking if the connection is lost due to a delay.
Interlining agreements differ from codeshare agreements in that codeshare agreements usually refers to numbering a flight with the airline's code (abbreviation) even though the flight is operated by another airline. However, codeshare relationships can affect whether an interline ticket (or e-ticket) can be issued, since both the codeshare marketing carrier and codeshare operating carrier must have interline agreements with all other carriers in the itinerary to allow a single ticket to be issued.