Top: total mass in form of prestellar cores as function of ambient cloud column density in Orion B (blue histogram). Bottom: distribution of background column densities for the 490 robust prestellar cores (blue histogram). The vertical dashed lines mark 3 and 7 mag. The overlaid gray histograms show the same distributions excluding the cores lying in the northern region L1622, which are at a more uncertain distance (see Sects. 1 and 3.5.2 for details).