Two possible geometrical constructions for relating group index n_g to phase index n: (a) with the dependence in wavelength λ, i.e., the spatial period of the wave, the tangent T₀(λ) to the phase index curve crosses the ordinate axis at the value n_g(λ₀) of the group index; (b) with the dependence in angular frequency ω, a double-slope line DS₀(ω) is drawn from where the tangent T₀(ω) to the phase index curve crosses the ordinate axis, and the group index n_g(ω₀) equates DS₀(ω₀).