Depolarization is the reduction in degree of polarization which occurs when a light beam interacts with surfaces or optical elements. Depolarization is a scrambling of polarization states. The depolarization of Mueller matrices is studied here by mapping the resultant degree of depolarization for all possible polarized incident states. Example Mueller matrices are constructed with a variety of different depolarization properties to demonstrate the different depolarization degrees of freedom. These properties can then be applied to understand measurements of depolarization. A classification is presented where depolarization is considered as occurring before diattenuation, in series with diattenuation, or following diattenuation.