Limits on the tensor-to-scalar ratio, r_0.002, as a function of n_S in the ΛCDM model at 95% CL, from Planck alone (grey area), or including BICEP2/Keck data 2014 (red) and BAO (blue). Constraints assume negligible running of the inflationary consistency relation, and the lines show the predictions of a number of models as a function of the number of e-folds, N_⋆, until the end of inflation. This can be compared with the middle panel in the top row of Fig. 14, which gives a temporal perspective.