Polarization amplitude rms as a function of frequency and astrophysical components, evaluated at a smoothing scale of 40′ FWHM. The green band indicates polarized synchrotron emission, and the red band indicates polarized thermal dust emission. The cyan curve shows the CMB rms for a ΛCDM model with τ = 0.05, and is strongly dominated by E-mode polarization. The dashed black lines indicate the sum of foregrounds evaluated over three different masks with f_sky = 0.83, 0.52, and 0.27. The widths of the synchrotron and thermal dust bands are defined by the largest and smallest sky coverages.