  We present and analyze a thermodynamical theory of rheology with single
internal variable. The universality of the model is ensured as long as the
mesoscopic and/or microscopic background processes satisfy the applied
thermodynamical principles, which are the second law, the basic balances and
the existence of an additional-tensorial-state variable. The resulting model,
which we suggest to call the Kluitenberg-Verh\'as body, is the
Poynting-Thomson-Zener body with an additional inertial element, or, in other
words, is the extension of Jeffreys model to solids. We argue that this
Kluitenberg-Verh\'as body is the natural thermodynamical building block of
rheology. An important feature of the presented methodology is that nontrivial
inequality-type restrictions arise for the four parameters of the model. We
compare these conditions and other aspects to those of other known
thermodynamical approaches, like Extended Irreversible Thermodynamics or the
original theory of Kluitenberg.
