  We have measured the optical linear polarization of quasars belonging to
Gpc-scale quasar groups at redshift z ~ 1.3. Out of 93 quasars observed, 19 are
significantly polarized. We found that quasar polarization vectors are either
parallel or perpendicular to the directions of the large-scale structures to
which they belong. Statistical tests indicate that the probability that this
effect can be attributed to randomly oriented polarization vectors is of the
order of 1%. We also found that quasars with polarization perpendicular to the
host structure preferentially have large emission line widths while objects
with polarization parallel to the host structure preferentially have small
emission line widths. Considering that quasar polarization is usually either
parallel or perpendicular to the accretion disk axis depending on the
inclination with respect to the line of sight, and that broader emission lines
originate from quasars seen at higher inclinations, we conclude that quasar
spin axes are likely parallel to their host large-scale structures.
