What is "interlining"

Interline \In`ter*line"\, v. t. [imp. & p. p. Interlined; p. pr. & vb. n. Interlining.] [Pref. inter- + line: cf. LL. interlineare, F. interlin['e]er, OF. entreligner.]

1. To write or insert between lines already written or printed, as for correction or addition; to write or print something between the lines of; as, to interline a page or a book.
   --Swift.

2. To arrange in alternate lines; as, to interline Latin and English.
   --Locke.

3. To mark or imprint with lines.

   A crooked wrinkle interlines my brow.
   --Marlowe.

Interlining \In`ter*lin"ing\, n. Correction or alteration by writing between the lines; interlineation.
--Bp. Burnet.

n. 1 A cloth lining between the outer and inner layers of a garment. 2 Correction or alteration by writing between the lines; interlineation. 3 The scheduling of vehicles to operate more than one route, or the selling of tickets for a trip across multiple carriers

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.

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.

Interline agreements are directional. For example, it may be possible for American Airlines to issue the ticket on an American- United itinerary but United might not be able to be issuer on the same itinerary.

Previously, only large network carriers such as United Airlines and Lufthansa would have electronic ticket interline agreements but the IATA mandate to eliminate paper tickets at the end of 2007 has changed this by forcing smaller carriers to implement electronic ticketing.

Smaller legacy carriers commonly have interline agreements with large network carriers that fly into their markets. Most newer low-cost airlines that only sell directly to consumers (and not through agencies or GDS systems) do not support interlining at all.

If no interline ticketing agreement exists, 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 are riskier 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 will sometimes display un-ticketable interline itineraries. Examples could be found previously on routes to Mexico involving the now defunct Aero California or may be currently found on routes to Indonesia involving Lion Air. These are displayed on Orbitz as "contact airline to buy".