The rescattered radiation becomes polarized since, as discussed in §3.8 temperature inhomogeneities, become anisotropies by projection, passing through quadrupole anisotropies when the perturbations are on the horizon scale at any given time. The result is a bump in the power spectrum of the -polarization on angular scales corresponding to the horizon at reionization (see Plate 1). Because of the low optical depth of reionization and the finite range of scales that contribute to the quadrupole, the polarization contributions are on the order of tenths of K on scales of few. In a perfect, foreground free world, this is not beyond the reach of experiments and can be used to isolate the reionization epoch [Hogan et al, 1982,Zaldarriaga et al, 1997]. As in the ISW effect, cancellation of contributions along the line of sight guarantees a sharp suppression of contributions at higher multipoles in linear theory. Spatial modulation of the optical depth due to density and ionization (see §4.3.4) does produce higher order polarization but at an entirely negligible level in most models [Hu, 2000a].