Example of a pair of fixed simulations. First and second panel: slices of fixed and paired reionization simulations (A and B) at z = 9 (with a mean neutral fraction of x_HI = 0.8). The colour map shows the differential brightness temperature between 0 and 50 mK. Third panel: 21 cm power spectra of the same paired simulations (blue and orange) along with those of 100 independent traditional simulations (grey lines). The mean power spectra of the traditional simulations and the two F&P simulations are shown as solid green and dotted red lines, respectively. For reference, the expected largest scale probed by SKA, k = 0.1 Mpc⁻¹, is marked with a vertical line.