Definition of angles. Here the line of sight is along the z′ axis (see Fig. 13), γ is the angle the magnetic field B makes with the plane of the sky, φ is the local polarization angle, and χ is the position angle of the plane of the sky projection B_⊥, both in the HEALPix convention, so counted positively clockwise from the north-south direction, while the IAU convention is anti-clockwise (Planck Collaboration Int. XIX 2014).