  This paper describes some generalities about spectro-interferometry and the
role it has played in the last decade for the better understanding of
circumstellar matter. I provide a small history of the technique and its
origins, and recall the basics of differential phase and its central role for
the recent discoveries. I finally provide a small set of simple interpretations
of differential phases for specific astrophysical cases, and intend to provide
a "cookbook" for the other cases.
