  The importance of charge reservoir layers for supplying holes to the CuO$_2$
planes of cuprate superconductors has long been recognized. Less attention has
been paid to the screening of the charge transfer by the intervening ionic
layers. We address this issue in the case of YBa$_2$Cu$_3$O$_{6+x}$, where CuO
chains supply the holes for the planes. We present a simple
dielectric-screening model that gives a linear correlation between the relative
displacements of ions along the $c$ axis, determined by neutron powder
diffraction, and the hole density of the planes. Applying this model to the
temperature dependent shifts of ions along the $c$ axis, we infer a charge
transfer of 5-10% of the hole density from the planes to the chains on warming
from the superconducting transition to room temperature. Given the significant
coupling of $c$-axis displacements to the average charge density, we point out
the relevance of local displacements for screening charge modulations and note
recent evidence for dynamic screening of in-plane quasiparticles. This line of
argument leads us to a simple model for atomic displacements and charge
modulation that is consistent with images from scanning-tunneling microscopy
for underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$.
