Constraints on ionization fraction during reionization. The allowed models, in terms of z_re and Δz, translate into an allowed region in x_e(z) (68% and 95% in dark blue and light blue, respectively), including the z_end> 6 prior here. Left: constraints from CMB data using a redshift-symmetric function (x_e(z) as a hyperbolic tangent with δz = 0.5). Centre: constraints from CMB data using a redshift-asymmetric parameterization (x_e(z) as a power law). Right: constraints from CMB data using a redshift-symmetric parameterization with additional constraints from the kSZ effect.