  Efforts to understand metallic behaviour have led to important concepts such
as those of strange metal, bad metal or Planckian metal. However, a unified
description of metallic resistivity is still missing. An empirical analysis of
a large variety of metals shows that the parallel resistor formalism used in
the cuprates, which includes T-linear and T-quadratic dependence of the
electron scattering rates, can be used to provide a phenomenological
description of the electrical resistivity in all metals, where these two
contributions are shown to correspond to the two first terms of a Taylor
expansion of the resistivity, detached of their physics origin, and thus, valid
for any metal. Here we show that the different metallic classes are then
determined by the relative magnitude of these two components and the magnitude
of the extrapolated residual resistivity. These two parameters allow to
categorize a few systems that are notoriously hard to ascribe to one of the
currently accepted metallic classes. This approach also reveals that the
T-linear term has a common origin in all cases, strengthening the arguments
that propose the universal character of the Planckian dissipation bound.
