Emergent Q/I profiles of Ca I 4227 at μ = 1, obtained from CRD, PRD–AA, and PRD–AD calculations in the FAL-C atmospheric model, in the presence of a height-independent horizontal (θ_B = π/2, χ_B = 0) magnetic field of 20 G. The reference direction for positive Stokes Q is parallel to the y-axis of the considered reference system (i.e., perpendicular to the magnetic field). In this geometry, the U/I profile vanishes, and is thus omitted.