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The main goal of this paper is to characterize limit key polynomials for a valuation $\nu$ on $K[x]$. We consider the set $\Psi_\alpha$ of key polynomials for $\nu$ of degree $\alpha$. We set $p$ be the exponent characteristic of $\nu$. Our first main result (Theorem 1.1) is that if $Q_\alpha$ is a limit key polynomial for $\Psi_\alpha$, then the degree of $Q_\alpha$ is $p^r\alpha$ for some $r\in\mathbb N$. Moreover, in Theorem 1.2, we show that there exist $Q\in\Psi_\alpha$ and $Q_\alpha$ a limit key polynomial for $\Psi_\alpha$, such that the $Q$-expansion of $Q_\alpha$ only has terms which are powers of $p$.

In this paper we present a matricial result that generalizes Hironaka's game and Perron transforms simultaneously. We also show how one can deduce the various forms in which the algorithm of Perron appears in proofs of local uniformization from our main result.