The advent of the jet age has considerably increased the need, especially for smaller air carriers, to cooperate. Cooperation is a means to reduce the costs of airline operation and in this case it could facilitate the costly changeover to jet aircraft operations. At the same time cooperation can strengthen the competitive—and sometimes also the air policy—position of the partners. Interchange of aircraft is a modest form of interairline cooperation, which is primarily geared to rationalization of airline operation, i.e. to improve the efficiency of operation by meeting an aircraft shortage experienced by one partner, and by increasing the utilization of the aircraft of the other partner.