  Let $R$ be an indecomposable root system. It is well known that any root is
part of a basis $B$ of $R$. But when can you extend a set of two or more roots
to a basis $B$ of $R$? A $\pi$-system is a linearly independent set of roots,
$C$, such that if $\alpha$ and $\beta$ are in $C$, then $\alpha - \beta$ is not
a root. We will use results of Dynkin and Bourbaki to show that with two
exceptions, $A_3 \subset B_n$ and $A_7 \subset E_8$, an indecomposable
$\pi$-system whose Dynkin diagram is a subdiagram of the Dynkin diagram of $R$
can always be extended to a basis of $R$.
