Genus curves of the H I density distribution at various epochs during reionization, labeled by redshift z and corresponding volume‐weighted mean neutral hydrogen fraction ﹩\overline{f}_{\mathrm{H}\,\,{}^{{\rm{\small I}}}}﹩. Following convention, we label the bottom axes with the quantity ν, which is related to the volume fraction f (top axes) by eq. (1) (see discussion in text). The dotted curves plotted in the top left and bottom right panels are the best‐fit theoretical curves for a Gaussian random‐phase distribution (eq. [4]). The vertical and horizontal dashed lines provide fiducial points for ﹩\nu =0﹩ and ﹩G_{s}=0﹩, respectively.