  Using real-space view of high harmonic generation (HHG) in solids, we develop
a physically transparent and gauge-invariant approach for distinguishing
intraband and interband HHG mechanisms. Our approach relies on resolving the
harmonic emission according to the separation between Wannier states involved
in radiative transitions. We show that the intra- and inter-band HHG emission
exhibit striking qualitative differences in their dependence on this separation
and can be clearly distinguished using the Wannier basis.
