  For a fixed irreducible polynomial $F$ we study the set $\mathcal V_F$ of all
valuations on $K[x]$ bounded by valuations whose support is $(F)$. The first
main result presents a characterization for valuations in $\mathcal V_F$ in
terms of their graded rings. We also present a result which gives, for a fixed
$\nu\in \mathcal V_F$ and a key polynomial $Q\in{\rm KP}(\nu)$, the maximum
value that augmented valuations in $\mathcal V_F$ can assume on $Q$. This value
is presented explicitly in terms of the slopes of the Newton polygon of $F$
with respect to $Q$. Finally, we present some results about Artin-Schreier
extensions that illustrate the applications that we have in mind for the
results in this paper.
