I think Comair mostly supplements Delta's service on some of the routes previously mentioned. Certain routes offered at certain times of the day might be supplemented better by being served by a smaller aircraft. An example is Delta/Comair's Columbus-Cincinnati route. Delta operates 727 and MD-80 aircraft on the route, and Comair also flies CRJ's and EMB-120's on the same route. I assume that the larger aircraft are used when passenger demand is higher, and the Comair aircraft are used when the demand is not as great.