Originally Posted by
Critic
That's a fair point. My only experience with a split itinerary (like the OP's) was DL-CO connection (back in the SkyTeam days). I had assumed that the common check-in/check-through was an alliance function, not a specific partnership function.
It's really weird. Even non-alliance carriers, they can sometimes do it. I had CO check me in through to US, and UA to CO, well before they were partners. I even had UA check me in through to MX once, though they were formerly partners.