Emitter’s and observer’s polarization bases. All vectors discussed here are expressed in the emitter’s frame. The direction of emission is K, while B_⊥ is the ambient magnetic field projected normal to K. The emitter’s frame polarization vector reads F = K × B = K × B_⊥. The vectors (e₁, e₂, e₃) = (F, −B_⊥, K) form the emitter’s orthogonal basis, naturally adapted for expressing synchrotron radiative transfer. The polarization basis of the observer (e_α, e_δ), corresponding to unit vectors in the east and north directions, has been parallel transported to the emitter’s frame. The vector e_w = −e_α is along the west direction such that (e_w, e_δ, K) forms the observer’s orthogonal triad. The emission EVPA is the angle χ_e = (e_δ, F) evaluated east of north lying between the observer’s and emitter’s bases. It is expressed by Eq. (21).