Contour maps of representative Balmer-emissivity-ratio surfaces constructed from Hα/Hβ, Hγ/Hβ, Hδ/Hγ, and Hϵ/Hδ, illustrating the qualitatively similar gradient structure in the ﹩\mathrm{log}\,{n}_{{\rm{e}}}﹩–﹩\mathrm{log}\,{T}_{{\rm{e}}}﹩ plane across different Balmer-line normalizations. The ratios are computed from interpolated Case B emissivities, plotted on a common color scale, and exhibit systematic variations of gradients, particularly along the ﹩\mathrm{log}\,{T}_{{\rm{e}}}﹩ direction in the low-﹩\mathrm{log}\,{n}_{{\rm{e}}}﹩ regime and along the ﹩\mathrm{log}\,{n}_{{\rm{e}}}﹩ direction in the low-﹩\mathrm{log}\,{T}_{{\rm{e}}}﹩ regime.