After publications by Emmerich and Korn <1987> and Carcione et al. <1988a, 1988b> authors who implemented realistic attenuation in the time-domain methods decided for either of two rheological models -- generalized Maxwell body (as defined by Emmerich and Korn) or generalized Zener body. Two parallel sets of papers and mathematical formalisms developed during the years. We have not found any comments on the other rheology. Therefore, we review both models and show that, in fact, they are equivalent. We also derive material-independent anelastic functions.