A passenger can sometimes buy one itinerary containing flights operated by two or more different airlines, check baggage through to a final destination and receive a single set of travel documents even though no single carrier operates the complete journey. That arrangement is made possible by interline relationships. IATA describes interlining as a framework under which airlines can accept passengers and cargo on each other’s services, with agreed rules covering documents, responsibilities and financial settlement. [1] [2]
Interlining should not be confused automatically with a codeshare or an airline alliance. A codeshare involves one airline placing its marketing code on a flight operated by another carrier, while an interline arrangement establishes the commercial and procedural ability for carriers to handle connecting traffic across their services. Airlines can interline without being members of the same global alliance, and an interline itinerary can contain flights marketed under their operating carriers’ own flight numbers. IATA’s Multilateral Interline Traffic Agreement, or MITA, provides one standard framework through which participating carriers establish those relationships. [1] [3]
Why airlines interline
An airline’s own network can serve only the destinations and frequencies it operates. Interlining allows a carrier to offer customers journeys beyond that network by combining its flight with another airline’s service. IATA describes new interline relationships as a way for airlines to increase connectivity, serve more origin-and-destination markets and create additional sales opportunities without each airline needing to operate every sector itself. [2]
The value is particularly clear on connecting journeys. One airline may provide the long-haul sector while another provides the regional feeder or onward connection. If the carriers have the necessary commercial and technical relationships, the itinerary can be constructed as one interline journey rather than two completely independent tickets. The exact customer protections and handling conditions depend on the ticket, applicable law, tariff and airline agreements, so interline should not be described as producing one universal set of rights in every market. [1]
MITA provides a common legal framework
IATA says MITA establishes the legal framework and describes responsibilities, liability provisions and general procedural obligations under which participating airlines can create interline partnerships. The current MITA manual contains passenger and cargo interline agreements and the basic rules carriers follow when collecting money and issuing documents for carriage on one another’s services. [1] [3]
The 2026 edition of IATA’s MITA manual is effective from 1 June 2026 to 31 May 2027. IATA lists participating carriers, multilateral agreements, cargo-claims arrangements, passenger and cargo service-charge agreements and information relating to clearing houses among the material contained in MITA. That illustrates how interlining extends beyond simply allowing a reservation system to display another airline’s flight. [3]
Bilateral interline relationships also exist
Not every airline relationship has to be created through the multilateral MITA structure. IATA also maintains information on bilateral interline electronic-ticketing agreements. Its MITA and BIETA data service distinguishes the multilateral agreements from bilateral ticketing relationships arranged between specific commercial airlines. [4]
IATA states that its bilateral interline e-ticketing data captures thousands of passenger and cargo ticketing agreements outside the multilateral framework. The existence of both models gives airlines flexibility: they can use a common industry framework where appropriate while also forming carrier-specific relationships. [4]
One ticket can contain several operating carriers
Electronic ticketing allows flight coupons for multiple sectors to be associated with one itinerary. In an interline journey, one airline may issue the ticket while another carrier provides part of the transportation. IATA’s passenger-service standards cover reservations, ticket issuance, passenger processing and interline carriage, providing standardised procedures that help different airline systems recognise and process the same journey. [5]
The issuing carrier and operating carrier therefore have different roles. The airline whose ticketing stock is used may collect the payment and issue the travel document, while operating carriers perform the individual flight sectors. Interline agreements establish the framework through which those carriers accept the relevant documents and later account for the revenue between themselves. [3]
The fare paid by the passenger has to be divided between airlines
If one customer pays for a journey operated by several carriers, the airlines need rules for allocating revenue. IATA states that airlines engaged in interlining depend on settlement agreements to help determine each airline’s revenue. Its interline framework and Prorate Agency support the processes used to allocate amounts between participating carriers. [1]
This financial process happens behind the passenger-facing ticket. The customer sees a total fare, taxes and charges according to the booking presentation, while the airlines’ commercial systems determine what value belongs to each participating carrier under the applicable agreements and proration rules. It is therefore possible for one airline to sell a journey while only operating one part of it. [1] [3]
Baggage needs its own interline process
Connecting baggage cannot be handled merely by issuing a multi-airline passenger ticket. Baggage tags, check-in messages, transfer procedures and carrier acceptance have to be standardised enough for the bag to move through multiple handling systems. IATA’s Passenger Services Conference Resolution Manual contains agreed standards for passenger and baggage check-in, ticketing and interline carriage. [5]
Whether a bag can actually be checked through on a particular itinerary depends on the airlines, airports, journey and applicable agreements. An interline relationship provides the commercial and procedural framework, but it should not be interpreted as a promise that every bag on every combination of carriers will always be through-checked under identical conditions. The passenger-facing rule remains the baggage arrangement confirmed for the specific itinerary. [5]
Reservations systems have to exchange status information
A multi-carrier journey depends on the airlines knowing whether the relevant segments are booked, ticketed, changed or cancelled. IATA’s passenger-services standards exist in part to provide common processes for reservations and passenger handling across carriers. Without interoperable data and agreed status conventions, one airline could not reliably sell or service a segment operated by another. [5]
Traditional interline technology grew around electronic tickets and passenger-name-record messaging, while IATA is now developing modern retailing models intended to simplify how offers and orders move between airlines. IATA’s Multilateral Interline Framework page describes that transition as part of its Future of Interline work. [1]
Interline does not automatically mean codeshare
A codeshare is a marketing arrangement in which a flight operated by one carrier can also be sold under another carrier’s flight code. Interline is broader in the sense that it provides the framework for accepting passenger or cargo traffic across carriers. IATA’s interline material focuses on carriage, documents and settlement rather than requiring participating carriers to place their codes on each other’s flights. [2]
The two arrangements can exist together. Airlines that codeshare commonly need interline capabilities to handle tickets, passengers and baggage across the partnership, but an interline agreement on its own does not necessarily create a marketing codeshare. Treating the terms as interchangeable obscures the different commercial functions they perform. [1]
Interline is not the same as alliance membership
Global alliances are broader commercial groupings that can involve frequent-flyer reciprocity, lounge arrangements, coordinated products and other cooperation. IATA’s interline frameworks are industry mechanisms available to participating airlines and are not limited to members of one alliance. Two carriers can therefore interline even when they belong to different alliances or to no global alliance at all. [2]
This is one reason interlining is important to global connectivity. It allows commercial cooperation to be built at the journey level without requiring every relationship to become a deep strategic alliance. IATA explicitly presents interlining as a way for airlines to expand reach and create connections across networks. [2]
Irregular operations require responsibility rules
If a journey involves several airlines, disruption raises practical questions about which carrier handles rebooking, document changes and other passenger-service actions. IATA says MITA describes responsibilities, liability provisions and general procedural obligations between participating airlines. Those rules create a contractual framework between carriers so that interline traffic does not depend on improvising every responsibility after disruption occurs. [1]
Passenger rights themselves can also arise from national or regional consumer law and from the conditions of carriage, so the existence of an interline agreement should not be described as the only legal source governing disruption. The narrower and verifiable point is that the airline-to-airline framework allocates procedural obligations and supports the handling of common interline situations. [3]
Interline cargo uses the same broad concept
IATA’s MITA is not limited to passenger traffic. The manual includes passenger and cargo interline agreements and specific cargo claims provisions. Cargo can therefore be transferred between airlines under standardised commercial frameworks in much the same way that passenger journeys can contain sectors operated by different carriers. [3]
The operational documents differ from passenger tickets, but the network logic is similar: one carrier does not need to fly the complete origin-to-destination routing if another participating carrier can provide a connecting sector and the required commercial agreement exists. IATA includes both passenger and cargo participation information in its interline data products. [4]
Settlement is essential because the carriers remain separate businesses
An interline itinerary may feel like one product to the customer, but the operating airlines remain independent companies with separate accounts. IATA therefore includes clearing-house and proration mechanisms within the wider interline ecosystem. Revenue collected by one party has to be accounted for and passed to the carrier entitled to the relevant share. [3] [1]
This financial layer is invisible at the airport but fundamental to the product. Without an agreed method for accepting documents and settling value, one airline would have little basis for transporting a passenger whose fare had been collected by another carrier. Interline agreements turn that commercial trust into a documented framework. [2]
Modern airline retailing is changing the technology
IATA is working on a future interline model aligned with airline retailing and offer-and-order concepts. Its Multilateral Interline Framework page describes the objective of creating simpler and more flexible engagement between parties as airline distribution moves beyond some of the traditional ticketing structures. The commercial need for airlines to cooperate remains, even as the technical representation of the customer order evolves. [1]
IATA’s 2026 Passenger Services Conference Resolution Manual includes standards linked to New Distribution Capability and One Order alongside traditional passenger and interline processes. That coexistence shows the industry in transition: established interline mechanisms continue to operate while airlines develop newer retailing technology. [5]
Why passengers often never notice the complexity
The purpose of standardisation is to hide much of the complexity from the customer. A passenger can search an origin and destination, buy a combined itinerary and receive consistent travel documents because airlines have agreed how reservations, tickets, baggage and settlement information are to be exchanged. IATA’s standards provide common processes so the customer does not need to understand the accounting relationship between every pair of carriers in the itinerary. [5]
That simplicity is conditional on the actual partnership. Two airlines appearing in the same booking search does not automatically prove that they have full interline baggage, ticketing and disruption-handling arrangements in every context. The travel product offered for the specific itinerary remains the authoritative guide for what the customer can expect. [4]
The engineering-like logic behind an interline ticket
An interline journey is effectively a system interface between airlines. Reservation data must be understood, travel documents must be accepted, baggage processes must align, responsibilities must be defined and money must be reconciled after travel. IATA’s MITA, bilateral agreement data and passenger-service standards provide structured mechanisms for solving those interfaces. [3] [5]
The result is one of aviation’s least visible pieces of infrastructure. The passenger sees a journey; behind it sit contracts, messaging standards, ticketing relationships and settlement rules connecting independent airlines. Interline agreements allow those separate carriers to behave like parts of one itinerary without requiring them to become one airline. [1] [2]
Verified Sources / References
- IATA — Multilateral Interline Framework
- IATA — Multilateral & Bilateral Interline Traffic Agreements
- IATA — 2026 Multilateral Interline Traffic Agreements Manual
- IATA — MITA & BIETA Agreement Data
- IATA — 2026 Passenger Services Conference Resolution Manual
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