Symmetry of the red and blue Lyα peak positions in the observed and modelled spectra. Panel a: the observed Lyα double-peak separation as a function of the blue and red Lyα peak positions: the distributions are similar for both peaks, the correlation is tighter for the blue peak. Panel b: same as panel a but for shell models; here the correlation is tighter for the blue peak, and the red and blue peaks are not symmetric. Panel c: same as panel a but measured with respect to the central Lyα trough; the correlations here are stronger than in panel a. Panel d: same as panel b but measured with respect to the central Lyα trough; the asymmetry between the red and blue peaks disappears.