  In underdoped cuprate high $T_{\mathrm{c}}$ superconductors, various local
orders and symmetry breaking states, in addition to superconductivity, reside
in the CuO$_2$ planes. The confinement of the CuO$_2$ planes can therefore play
a fundamental role in modifying the hierarchy between the various orders and
their intertwining with superconductivity. Here we present the growth of
$a$-axis oriented YBa$_2$Cu$_3$O$_{7-\delta}$ films, spanning the whole
underdoped side of the phase diagram. In these samples, the CuO$_2$ planes are
confined by the film thickness, effectively forming unit-cell-thick
nanoribbons. The unidirectional confinement at the nanoscale enhances the
in-plane anisotropy of the films. By X-ray diffraction and resistance vs
temperature measurements, we have discovered the suppression of the
orthorhombic-to-tetragonal transition at low dopings, and a very high
anisotropy of the normal state resistance in the $b$-$c$ plane, the latter
being connected to a weak coupling between adjacent CuO$_2$ nanoribbons. These
findings show that the samples we have grown represent a novel system,
different from the bulk, where future experiments can possibly shed light on
the rich and mysterious physics occurring within the CuO$_2$ planes.
