  In recent years a significant amount of research in quantum optics has been
devoted to the analysis of atomic three-level systems and for many physical
quantities the same effects have been predicted for different configurations.
These configurations can be split into essentially two classes. One for which
the system contains a metastable state and another where the system has two
close-lying levels and coherence effects become important. We demonstrate when
and why for a wide range of parameters these two classes are in fact equivalent
for many important physical quantities. A unified picture underlying a large
body of work on these categories of atomic three-level systems is presented and
applied to some examples.
