  We discuss the concept of local thermodynamical equilibrium in relativistic
hydrodynamics in flat spacetime in a quantum statistical framework without an
underlying kinetic description, suitable for strongly interacting fluids. We
show that the appropriate definition of local equilibrium naturally leads to
the introduction of a relativistic hydrodynamical frame in which the
four-velocity vector is the one of a relativistic thermometer at equilibrium
with the fluid, parallel to the inverse temperature four-vector \beta, which
then becomes a primary quantity. We show that this frame is the most
appropriate for the expansion of stress-energy tensor from local
thermodynamical equilibrium and that therein the local laws of thermodynamics
take on their simplest form. We discuss the difference between the \beta frame
and Landau frame and present an instance where they differ.
