However, this is a very key point in the NW-CO agreement. Neither carrier is permitted to sell seats on non-stop codeshare flights that originate or terminate in the other carrier's hub cities (defined as DTW, MSP, MEM for NW and EWR, IAH, CLE for CO). Hence, if you are flying EWR-BOS, you have to fly on a CO code if you want a nonstop, since NW cannot sell that ticket. They can however sell something like RDU-EWR-BOS on the same plane.