In this paper, we review the set of rules specific to the calculation of the
imaginary part of a Green's function at finite temperature in the real-time
formalisms. Emphasis is put on the clarification of a recent controversy
concerning these rules in the "1/2" formalism, more precisely on the issue
related to the interpretation of these rules in terms of cut diagrams, like at
T=0. On the second hand, new results are presented, enabling one to calculate
the imaginary part of thermal Green's functions in other formulations of the
real-time formalism, like the "retarded/advanced" formalism in which a lot of
simplifications occur.Comment: 25 pages, LaTeX file with "article" style, 6 postscript figures
included by \epsfbo
