Polarisation of double notches in the presence of two polarisation modes. a) Intensity and polarisation fraction for both modes (⊥-mode: solid line; ∥-mode: dashed line). Π_⊥ is indistinguishable from Π_∥ = 1, hence we plot 100(Π_⊥−0.99). b) Total intensity (thick solid line) and the total Π (thin line). c) Total PA ψ (thick solid line) overplotted on the net PA of the X mode (ψ_⊥, thin solid line), and the reference ψ_B of the RVM model (dotted line). The contribution of the O-mode increases the off-RVM amplitude of the total PA. d) Same as in c, but for the increasing contribution of the O-mode: , 0.5, 0.55, and 0.6. The last case (thin line with spikes) undergoes the orthogonal-mode jumps. The unspecified parameters are the same as in Fig. 6.