The transfer function of a system is defined as the output divided by the input. It consists of a gain and a phase element that can be plotted separately in a Bode plot. The gain around a closed loop system is the product of the gains of all the elements around the loop. In a Bode plot, the gain is plotted logarithmically. Since the product of two numbers is their logarithmic sum, their gains can be summed graphically. The phase of the system is the sum of all phase shifts around the loop.