Demonstration of the effect of 3D structure in the Serkowski relation, using analytical models for P - λ and θ-λ curves. In each case, the solid black curve corresponds to the combined model and the dashed red and dotted blue lines correspond to individual clouds. Left column: simple case of one cloud (Eq. (1)). Middle left column: example with two clouds in the LOS, in which both the combined P-λ, and θ-λ seem to follow the Serkowski relation. Middle right column: example with two clouds in the LOS, in which the combined P-λ seems to follow the Serkowski relation, but θ - λ not. Right column: Example with two clouds in the LOS in which both P-λ and θ - λ profiles deviate from Serkowski expectations. The parameters for each model are: Left: P_max = 1%, λ_max = 0.6μιη. θ = 30°. Middle left: P_max1 = 1.5%, λ_max1 = 0.55 μm, θ₁ = 30°, P_max2 = 0.7%, λ_max2 = 0.5 μm, θ₂ = 35°. Middle right: P_max1 = 0.7%, λ_max1 = 0.5 μm, θ₁ = 0°, P_max2 = 0.8%, λ_max2 = 0.7 μm, θ₂ = 60°. Right: P_max1 = 1.3%, λ_max1 = 0.4 μm, θ₁ = 0°, P_max2 = 1.4%, λ_max2 = 0.8 μm, θ₂ = 75°. For all clouds, we have used K = 1.68·λ_max (Wilking et al. 1980).