For simple systems, there may be as few as one or two degrees of freedom. One degree of freedom occurs when one has an autonomous ordinary differential equation in a single variable, d y / d t = f ( y ) , with the resulting one-dimensional system being called a phase line, and the qualitative behaviour of the system being immediately visible from the phase line. The simplest non-trivial examples are the exponential growth model /decay (one unstable/stable equilibrium) and the logistic growth model (two equilibria, one stable, one unstable).