  We present a study of the continuum polarization over the 400--600 nm range
of 19 Type Ia SNe obtained with FORS at the VLT. We separate them in those that
show Na I D lines at the velocity of their hosts and those that do not.
Continuum polarization of the sodium sample near maximum light displays a broad
range of values, from extremely polarized cases like SN 2006X to almost
unpolarized ones like SN 2011ae. The non--sodium sample shows, typically,
smaller polarization values. The continuum polarization of the sodium sample in
the 400--600 nm range is linear with wavelength and can be characterized by the
mean polarization (P$_{\rm{mean}}$). Its values span a wide range and show a
linear correlation with color, color excess, and extinction in the visual band.
Larger dispersion correlations were found with the equivalent width of the Na I
D and Ca II H & K lines, and also a noisy relation between P$_{\rm{mean}}$ and
$R_{V}$, the ratio of total to selective extinction. Redder SNe show stronger
continuum polarization, with larger color excesses and extinctions. We also
confirm that high continuum polarization is associated with small values of
$R_{V}$.
  The correlation between extinction and polarization -- and polarization
angles -- suggest that the dominant fraction of dust polarization is imprinted
in interstellar regions of the host galaxies.
  We show that Na I D lines from foreground matter in the SN host are usually
associated with non-galactic ISM, challenging the typical assumptions in
foreground interstellar polarization models.
