Pairs (Ω₂, i₂) that vanish the ω₂ quadrupole precession rate are illustrated by a blue curve for different values of the eccentricity e₁ associated with the inner perturber, which is specified at the top left corner of every panel. For each scenario, the separatrix of the system, which is computed up to the quadruple level of the secular approximation, is represented by a red curve, while the red shaded region illustrates the pairs (Ω₂, i₂) that lead to the orbit-flipping quadrupole resonance.