Top: marginalized constraints on the optical depth to reionization from lowE alone, assuming different models of reionization and different priors over the model parameters. Only reionization parameters are varied here, with A_se^−2τ and other cosmological and instrumental parameters held fixed at their best-fit values from Planck TT,TE,EE. The solid lines use a flat prior on τ, while the dashed line uses a flat prior on the knot amplitudes; the difference between the green lines is an example of the level to which these constraints depend on the choice of prior. Bottom: constraints from different data sets on the optical depth assuming the TANH model and a flat τ prior (the cases that include high-ℓ data are indicated by dot-dashed lines and also marginalize over ΛCDM parameters, as opposed to fixing them). The Planck TT,TE,EE+lowE line is shown without the lower bound due to measurements of the Gunn-Peterson trough, as a reminder that this bound is applied only in this section, resulting in some small extra shifts in the central values of quoted constraints between this section and the remainder of the paper.