Spatial coupling between lines of sight in a 3D (left) and in a 1.5D (right) model atmosphere. In 3D, the radiation field at a given point results from radiation transfer from different regions of the domain, both in the vertical and in the horizontal directions. In 1.5D, the horizontal variation is neglected. The 1.5D model can be sufficiently accurate in some cases but, in general, it often leads to serious errors in calculation of the line scattering polarization.