  We study canonical bases of a subalgebra ${\bf A}={\mathbb
K}[\![f_1,\dots,f_s]\!]\subseteq {\mathbb K}[\![x_1,\dots,x_n]\!]$ over a field
${\mathbb K}$, and we associate with ${\bf A}$ a fan called the canonical fan
of $\bf A$. This generalizes the notion of the standard fan of an ideal.
