  For any choice of a basis $\cal A$ the free group $F_N$ of finite rank $N
\geq 2$ can be canonically identified with the set $F(\cal A)$ of reduced words
in $\cal A\cup \cal A^{-1}$. However, such a word $w \in F(\cal A)$ admits a
second interpretation, namely as cylinder $C^1_w \subset \partial F_N$. The
subset of $\partial F_N$ defined by $C^1_w$ depends not only on the element of
$F_N$ given by the word $w$, but also on the chosen basis $\cal A$. In
particular one has in general, for $\Phi \in \Aut(F_N)$: $$\Phi(C^1_w) \neq
C^1_{\Phi(w)}$$ Indeed, the image of a cylinder under an automorphism $\Phi \in
\Aut(F_N)$ is in general not a cylinder, but a finite union of cylinders:
$$\Phi (C^1_w)=C^{1}_U := \bigcup_{u_i \in U} C^1_{u_i}$$ In his thesis the
first author has given an efficient algorithm and a formula how to determine
such a (uniquely determined) finite {\em reduced} set $U = U(w) \subset F_N$.
We use those to define the dual automorphism $\Phi_{\cal A}^*$ by setting
$\Phi_{\cal A}^*(w) = U(w)$.
  \smallskip \noindent {\bf Theorem:} {\it For any $\Phi \in \Aut(F_N)$ there
are at most 2N distinct finite subsets $U_i \subset F_N$ such that for any $w =
y_1 ... y_r \in F_A$ there is one of them, say $U_{i(w)}$, with $$\Phi_{\cal
A}^*(w) = \Phi(w) U_{i(w)}\, ,$$ and $U_{i(w)}$ depends only on the last letter
$y_r \in \CA \cup \CA^{-1}$. Furthermore, the seize of each $U_{i}$ is bounded
by $2^t$, where $t \geq 0$ is the number of Nielsen automorphisms in any
decomposition of $\Phi$ as product of basis permutations, basis inversions and
elementary Nielsen automorphisms.}
