Schematic showing the inferred distributions of species in a diffuse cloud, here represented by a plane‐parallel slab from the cloud surface to its denser regions. Different species are distributed according to gas density in the diffuse molecular gas (see text in § 7.1). Dashed and solid vertical lines indicate the inner and outer boundaries of species distributions, respectively. In some low‐density clouds, CN and CH are not present. If the density is even lower, Ca II is the only one among these species present in the cloud. The spacing between hash marks symbolizes (but not to scale) changes in gas density.