Center-to-limb variation of the Ca I 4227 line-center fractional linear polarization Q/I (upper panel) and U/I (lower panel) obtained from CRD, PRD–AA, and PRD–AD calculations in the FAL-C atmospheric model, both in the absence (dotted curves) and in the presence (solid curves) of a magnetic field. The dotted curves for U/I are equal to zero. In the magnetic case, a height-independent horizontal (θ_B = π/2, χ_B = 0) magnetic field of 20 G is considered. 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).