Measurement of the total column density of neutral atomic hydrogen at z = 2.69 towards J 1237 + 0647. The solid line represents the best one-component fit to the data. The centroid of the profile is indicated by a dotted vertical line. Uncertainties on the column density measurement are illustrated by the shaded area. The short tick marks indicate the position of the three H₂-bearing components, the reddest of which also features CO and HD absorptions. The origin of the velocity scale for this figure and all following ones is defined at the position of the CO-bearing component at z_abs = 2.68957.