  Electromagnetically Induced Transparency (EIT) and Autler-Townes (AT) are two
phenomena that could be featured by a variety of three-level atomic systems.
The considered phenomena, EIT and AT, are similar "looking" in the sense that
they both are characterized by a reduction in absorption of a weak field in the
presence of a stronger field. In this paper, we explicitly set the threshold of
separation between EIT and AT in a unified study of four different three-level
atomic systems. Two resonances are studied and compared in each case. A
comparison of the magnitudes of the resonances reveals two coupling field
regimes and two categories of three-level systems.
