The mechanical compliance and modulus retardation/relaxation functions are examined in terms of a general behaviour which contains more than one process. An analytical approach to the transformation in the anelastic response between the compliance and the modulus is derived and applied to a cooperative model of relaxation behaviour. In particular it is shown that mechanical viscoelasticity is equivalent to the anomalous low frequency dispersion process that has been observed in dielectrics containing quasifree charges. Comparison with published experimental data over a wide range of solid materials shows the validity of the cooperative model to mechanical relaxation.