The homogeneity principle states that the output of a linear system is always directly proportional to the input, so if we put twice as much into the system we will, in turn, get out twice as much. For example, if I pay 50 dollars for a hotel room, for which I will get a certain quality of service, this principle states that if I pay twice as much I will then get an accommodation service that is twice a good. When we plot this on a graph the result will be a straight line. Along with the additivity principle, the homogeneity principle forms what are called the superposition principles that define linear systems.