﹩68.3 \% ﹩ and ﹩95.5 \% ﹩ credible regions for ﹩{c}_{\mathrm{vis}}^{2}﹩ and c²_s, showing the strong negative correlation between these parameters. These contours are for the Λ-GDM run for Planck+Planck lensing. The red (dashed) lines denote lines of constant ﹩{c}_{s}^{2}+\tfrac{8}{15}{c}_{\mathrm{vis}}^{2}﹩, showing that the degeneracy is well fit by the ﹩{k}_{{\rm{d}}}﹩ parameter as explained in the text.