Ratio M₂/M₁ of the fit coefficients given in Tables 8 and 15 against rotation period. Open circles: stars showing ⟨ B_z ⟩ reversal; dots: stars in which ⟨ B_z ⟩ has a constant sign; open triangle: star for which no ⟨ B_z ⟩ measurements exist. For a fraction of the stars, the error bar is shorter than the size of the representative symbol. The horizontal (solid) line corresponds to the stars for which the curve of variation of ⟨ B_z ⟩ does not significantly depart from a sinusoid (M₂ = 0). The two vertical (dashed) lines emphasise the tendency for anharmonicity of the mean field modulus variations to be restricted to rotation periods shorter than P_rot = 100 d or greater than P_rot = 1000 d.