Originally Posted by jsloan

OP: The single most important question is, what's the marketing carrier of your first flight? That is, are you flying TK123 or flight UA8123 operated by TK? That indicates whose baggage policies should prevail.

The receipt -- which, to be clear, is often wrong despite the fact that each incorrect receipt is a DOT violation -- indicates that TK originally chose to defer to UA, as the Most Significant Carrier for the journey, which is preferred under IATA rules and permitted (but not required) by the DOT. The second email indicates that TK is no longer deferring to UA, which is contrary to IATA policy but allowed by the DOT. I don't actually know whether or not there's any regulation that would prohibit TK from changing its mind about deferring to UA, although it's certainly against the spirit of the DOT regulation. None of this is even possible if the first flight is a UA code, which is why it's important to know that detail.

Regardless, TK includes baggage with every J ticket. The only real question is whether they'd apply their intra-Europe rule (30 kg total) or their to-US rule (2x23 kg). I would expect the latter, but I'm not entirely sure.