  The main result of this paper is that for any norm on a complex or real
$n$-dimensional linear space, every extremal basis satisfies inverted triangle
inequality with scaling factor $2^n-1$. Furthermore, the constant $2^n-1$ is
tight. We also prove that the norms of any two extremal bases are comparable
with a factor of $2^n-1$, which, intuitively, means that any two extremal bases
are quantitatively equivalent with the stated tolerance.
