  The stable set polytope of a graph $G$, denoted as STAB($G$), is the convex
hull of all the incidence vectors of stable sets of $G$. To describe a linear
system which defines STAB($G$) seems to be a difficult task in the general
case. In this paper we present a complete description of the stable set
polytope of ($P_6$,triangle)-free graphs (and more generally of
($P_6$,paw)-free graphs). For that we combine different tools, in the context
of a well known result of Chv\'atal \cite{Chvatal1975} which allows to focus
just on prime facet-inducing graphs, with particular reference to a structure
result on prime ($P_6$,triangle)-free graphs due to Brandst\"adt et al.
\cite{BraKleMah2005}. Also we point out some peculiarities of new
facet-inducing graphs detected along this study with the help of a software.
