I tried to find a workable answer using the search tool, but didn't find anything directly on point. Apologies if this has already been answered...

I searched travelocity recently for a flight from BOS>BNA. The best fare that came up was on NW metal with CO codeshares for both flights (connecting in DTW I believe). When searching CO.com, this itinerary was impossible to pull up. After calling reservations, I was told that CO would not ticket an itinerary because all of the flights were codeshares on NW. Is this correct? Can anyone guide me to a resource regarding codeshare rules for CO?

Thanks in advance...