  In this paper we present characterizations of the sets of key polynomials and
abstract key polynomials for a valuation $\mu$ of $K(x)$, in terms of
(ultrametric) balls in the algebraic closure $\overline K$ of $K$ with respect
to $v$, a fixed extension of $\mu_{\mid K}$ to $\overline K$. In particular, we
show that the ways of augmenting of $\mu$, in the sense of Mac Lane, are in
one-to-one correspondence with the partition of a fixed closed ball
$B(a,\delta)$ associated to $\mu$ into the disjoint union of open balls
$B^\circ(a_i,\delta)$, modulo the action of the decomposition group of $v$. We
also present a similar characterization for the set of limit key polynomials
for an increasing family of valuations of $K(x)$.
