Constraints on the free electron fraction, x_e(z), from lowE alone, with A_se^−2τ and other cosmological and instrumental parameters held fixed to their best-fit values from Planck TT,TE,EE, and with a flat prior on τ. The shaded bands are middle 68th and 95th percentiles (note that this does not correspond exactly to confidence intervals). The FlexKnot constraints show that any non-zero component of reionization above a redshift of about 15 is highly disfavoured.