  The phenomenology of the beta relaxation process in the shear-mechanical
response of glass-forming liquids is summarized and compared to that of the
dielectric beta process. Furthermore, we discuss how to model the observations
by means of standard viscoelastic modeling elements. Necessary physical
requirements to such a model are outlined, and it is argued that physically
relevant models must be additive in the shear compliance of the alpha and beta
parts. A model based on these considerations is proposed and fitted to data for
Polyisobutylene 680.
