Originally Posted by
cordelli
The rules of the first airline you check in with will usually apply for all the flights on the same reservation.
Originally Posted by
mahasamatman
As far as I've always heard, the carrier you check in with (i.e., the first segment) determines the baggage allowance.
This is not
supposed to be true anymore - I'm not going to dig up the link right now, but there are new (this year, at least) IATA regs this year (to which UA is party) to the effect of baggage rules of the "Most Significant Carrier" (almost always the overwater or between-region carrier) will apply. I've done this on AA and it was automated (to my great surprise), but on UA/CO... who knows what the bag-tagger may try to do.