If you've ever seen a PNR with two or three different airlines on it for what is plainly one trip, you've already run into interlining, even if nobody gave it a name. It sounds like a back-office detail, but it decides whether a client's bags are checked through, who rebooks them when a flight is late, and which airline you call when something goes wrong. This guide explains what it is, how it works behind the ticket, and how to tell it apart from codeshares and DIY connections.
What Is Interlining?
Interlining is a voluntary commercial agreement between airlines to handle passengers on itineraries that involve more than one carrier. One airline sells and issues a single ticket covering flights operated by several airlines. Each carrier accepts that ticket for its own flight, and baggage and check-in are handled as one journey.
The practical result: a passenger can fly an outbound leg on one airline and a connecting flight on another, with bags tagged to the final destination, one boarding-pass process, and a single ticket tying it together. They don't collect bags and queue at a second check-in desk. Many agents blur this with "separate tickets", which is the opposite situation, covered below.
How It Works Behind the Ticket
As an agent you mostly see the front end, but knowing the money flow explains several rules you'll meet.
- One carrier issues the ticket. This is the validating (or plating) carrier. It collects the fare, either directly or through an agent, and agents remit via ARC in the US or BSP in many other markets.
- The other carriers get paid for their share. The operating airline for each leg invoices the issuing carrier, usually through the IATA Clearing House, for its portion of the fare.
- Commission follows the ticket. The issuing carrier is generally the one that pays agency commission, calculated on the full fare.
- Which carrier should issue is governed by rules and the airlines' own agreements. For international itineraries it is commonly the first international carrier, with exceptions such as when that leg is a codeshare.
It's a simplified picture. Prorate calculations, taxes and settlement differ by market and agreement, and you don't need to run them yourself, but it explains why the validating carrier is the one you'll deal with on refunds and reissues.
The Agreements Behind It
Interlining only works where airlines have agreed to it. IATA's Multilateral Interline Traffic Agreement (MITA) is a single agreement that sets the legal framework, with responsibilities, liability provisions and procedural obligations, for IATA and non-IATA member airlines that want to create partnership agreements. Airlines can also sign bilateral e-ticketing agreements (IATA's BIETA data covers thousands of these) that sit outside MITA. IATA says interline today happens more often through codeshares, joint ventures and alliances than through multilateral arrangements alone, and that its members are exploring ways to reinvigorate the framework. You can read IATA's description on its interline framework page.
For an agent the takeaway is simple. An interline ticket is only possible between airlines with a working agreement, and that agreement may only run one way: one airline may be allowed to issue tickets on another's flights without the reverse being true. A GDS will usually refuse to price an itinerary that isn't permitted.
How Interlining Shows Up in a PNR
When a PNR includes interlined segments, you'll typically see different two-letter airline codes across the segment lines. For illustration (the flight numbers and times are made up):
1.KHAN/ALI MR 2 PK 302 Y 07SEP 1 LHEDXB SS1 1000P 1230A/08SEP 3 EK 001 Y 08SEP 2 DXBLHR SS1 830A 100P 4 BA 444 Y 08SEP 2 LHRORD SS1 430P 700P
This itinerary involves three airlines: PK (Pakistan International Airlines), EK (Emirates) and BA (British Airways), on one journey from Lahore to Chicago. The GDS PNR holds the whole trip, even though each airline keeps its own record for its own segments. Which airline is the validating carrier is recorded with the fare and ticket details, not on the segment lines, so you won't see it by reading the segments alone.
Notice that a PNR with several airlines doesn't prove the trip is on one ticket. The segment lines look identical whether the client holds one interline ticket or three separate ones. You find out when you price and ticket it.
Interlining vs Codeshare vs Alliance vs Self-Connect
These terms get mixed up constantly:
| Term | What It Means | One ticket? |
|---|---|---|
| Interlining | Airlines agree to accept each other's passengers and baggage on a multi-carrier itinerary | Yes |
| Codeshare | One airline sells seats on a flight actually operated by a different airline, under its own flight number | Can be sold on one ticket or as part of an interline itinerary |
| Alliance or joint venture | Groups of airlines with broad cooperation: connections, loyalty, often codeshares | Usually, within the group |
| Self-connect (virtual interlining) | Separate tickets on separate airlines, combined by the traveller or a third-party seller | No |
A codeshare flight shows an airline code that doesn't match who operates the plane, and the PNR will often include a note such as "OPERATED BY [other airline]". Codeshare and interline are different arrangements, but they interact: for an interline ticket to work, both the marketing and operating carriers generally need agreements with every other carrier on the itinerary.
One Interline Ticket vs Two Separate Tickets
This is the choice that matters most to your client's day.
| One interline ticket | Separate tickets (self-connect) | |
|---|---|---|
| Baggage | Normally checked through to the final destination | Usually collected and re-checked at the connection |
| Missed connection | The airlines may rebook the passenger, depending on their rules and agreements | Typically the passenger's problem: the second airline owes nothing for a late first flight |
| Check-in | Often one process for the whole journey | Separate check-in with each airline |
| Transit | Usually smoother airside | May require entering the country, with visa implications |
| Price | Sometimes higher | Sometimes cheaper, especially with low-cost carriers |
Separate tickets aren't inherently bad. Within a visa-free area such as Schengen, with a generous connection time, they can be a sensible way to save money. They're risky on tight connections, with checked bags, or where the second leg is critical, like a cruise departure.
Many low-cost carriers sell only through their own channels and don't interline at all. A missed connection on such a flight is the passenger's own problem to solve, so flag it clearly when a client wants to combine a no-frills carrier with a long-haul flight.
A Worked Scenario
Imagine an agent, Dana, asked to get a client from Lahore to Chicago. She prices two options. The first is the three-airline itinerary above on a single ticket. The second is a cheaper pair of tickets, a low-cost flight to Dubai and then a separate long-haul ticket onwards. Before quoting, Dana asks four questions. Is it one ticket or several? What happens to the bags at the connection? How long is the connection, and is it long enough if the first flight runs late? Does the client need airside transit, and are visa rules a problem? On the cheaper option, the answers are separate tickets, bags re-checked, a six-hour connection and a possible entry stamp. She quotes both with the trade-offs written out, and lets the client choose with full information. Quoting only the lower price, then having the client stranded in Dubai, would be the worst outcome.
A Checklist Before You Sell a Multi-Airline Itinerary
- Confirm whether it is a single ticket. Price it as one itinerary and see what the GDS accepts.
- Check the connection time against the airports and any terminal change, especially if a bag recheck is involved.
- Confirm baggage rules in the fare and ticket data rather than assuming a single allowance applies. Which airline's allowance counts depends on the carriers and the itinerary.
- Check marketing and operating carriers on every segment, and read any "operated by" notes.
- Check visa and transit requirements for each connection point.
- Tell the client, in writing, what happens if a connection is missed.
When Something Goes Wrong
On a single interline ticket, if one airline's delay causes a missed connection, the carriers may rebook the passenger depending on their agreements, though the process is more involved than on a single-airline booking. The practical order is: contact the airline that operated the delayed flight, keep the ticket number and record locators to hand, and involve the validating carrier for anything that needs a reissue. ARC has noted that once the validating carrier's own segments have been flown, its ticketing database may be unable to find the PNR, so you may need to contact both airlines.
On separate tickets, the second airline has no obligation to the passenger for a delay on the first, and the passenger may have to buy a new ticket. Rules also vary by country and by airline. Regulations on passenger compensation, for example, may treat a single booking differently from separate ones, so check the rules that apply to your client's route, and treat anything here as general information rather than legal advice.
Interlining and the Newer Distribution World
Interline arrangements are also being rebuilt for NDC. Lufthansa Group has described interline NDC work with partners including United, Air Canada, Singapore Airlines and Qantas. For you, that means interline itineraries sold through NDC may behave differently when you service them. See our NDC guide.
Spotting Interlined Segments Quickly
To check a PNR, look at whether the airline code changes between segments while the trip is clearly one connected journey: same passenger, consecutive dates, connecting cities. Then check the ticketing record to see whether it was issued as one ticket. A converter that displays each segment's airline and route side by side, like PNRGenius's free PNR converter, makes the first part easier than scanning raw text. Our guides to what a PNR is and to status codes cover the rest of what you'll see on those lines.
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