Left: joint 2D histograms of polarization fraction p and column density N_H. Red dashed lines show the location of p_max = 19.8% from PlanckXIX. The green lines follow the maximum C_bin as a functionof N_H, and the blue lines represent the weighted mean of p. Right: sample maps of p. The polarization fractions are weakened in the galactic midplane, which is seen in the 2D histograms as a growing distribution of low p at high N_H. For R_max = 1 kpc, see Figs. 13 and 14.