  In this paper, we discuss five parameters that indicate the inhomogeneity of
a stack of parallel isotropic layers. We show that, in certain situations, they
provide further insight into the intrinsic inhomogeneity of a Backus medium, as
compared to the Thomsen parameters. Additionally, we show that the Backus
average of isotropic layers is isotropic if and only if $\gamma=0$. This is in
contrast to parameters $\delta$ and $\epsilon$, whose zero values do not imply
isotropy.
