In summary, these systems represent the framework in which the air transport market works. The market consists of a supply system and a demand system. The supply system consists of all partners along the supply chain that deliver to airlines and airports. On the demand side, there are consumers, like leisure and business customers, tour operators and travel sources that pay for an air service. Airlines generate their revenues from the market. The whole supply system is being paid from these revenues. By this the airlines are the most important factor in the supply system, in the aviation market and for whole aviation system. Figure 2.8 illustrates the aviation system.