Some precessing ellipses of the x₁ family (black orbits) together with the precessing ellipses of the x₂ family (red orbits) that correspond to the same radii r_c for density of the spiral potential ρ₀ = 30 × 10⁷ M_⊙ kpc⁻³ and for a pattern speed Ω_sp = 15 km s⁻¹ kpc⁻¹. The main axes of the x₂ ellipses are perpendicular to the main axes of the x₁ family and do not support the spiral density wave.