Governing carrier on tickets
 

Is it still that the governing carrier on a flight ticket is always the airline that is operating the longest sector on the ticket, or has that definition been changed in more recent decades?

Where available, PTAs seem to be issuable by carriers who are either the governing carrier or operators of the first international sector on a ticket.

The governing carrier on a ticket is the one whose fare basis is used for issuing the ticket. A carrier may not publish a fare basis that requires travel wholly outside of its own lines. If the fare basis is a YY basis then the governing carrier is the one whose stock is used to plate the ticket.

Why not? It happens all the time (sort of) with codeshares; I have flown tickets on CO stock that are entirely XE operated.

Outside of code shares, what rule(s) prevent carrier X from publishing a fare basis that requires travel solely on another airline? What about ticket stock: could CO issue a ticket on 005 stock that uses only OAL fare basis codes? This must be a frequent event in the area of reward tickets.

(I'm sufficiently knowledgeable to be dangerously clueless).

"Own lines" for the purpose of ticketing is considered to be any flight offered under the designator "CO", whether codeshare or franchise or otherwise.

Off the top of my head, MITA Article 2 Section 2.3 - "Multilateral Interline Traffic Agreement - Furnishing of Tariffs" governs this on a basic level. I am sure the Tariffs Manual has something in more detail, but I'm not going to pull that out to check.

Yes, Continental may issue a ticket on their own stock that uses only OAL fare basis codes. However, CO does not permit its agents to do so. To quote from the "Continental Booking and Ticketing Policy - BSP" - "all tickets issued on Continental (005) stock must include a Continental flight segment or a CO marketing flight operated by a codeshare partner under a CO flight number".

There are companies like APG-Heli Air Monaco (YO/747) and HahnAir (HR/169) whose entire business models are based upon being third party ticketing platforms to facilitate interline relationships which might otherwise not exist. This is especially useful in the modern world where IET can be a royal pain to coordinate.