Idealized illustration of different processes that can shape the ISM abundance patterns. The observed abundances are on the y-axis, and the tendency of each element to deplete into dust grains is on the x-axis. (a) If no dust or other processes that may cause additional deviations are at play, the observed abundances reflect the actual metallicity of the gas. (b) If there is dust, the refractory elements show lower abundances because they are depleted. The more dust, the steeper the slope of the linear relation. The y-intercept (at no depletion) of the solid line is the total (gas + dust) metallicity. (c) Deviations from the linear relation can be observed in the gas for specific elements, in this example α-element enhancement and Mn underabundance due to nucleosynthesis of recent core-collapse SNe. The dashed line shows a fit to the α-element data only. In this illustration Zn (and other elements, e.g. P) is excluded from determination of the linear fit, and is only shown here with the Fe-group elements. Any deviations can be observed in the data. (d) The presence along the line of sight of a mix of two gas components (empty circles) with different metallicities and amounts of dust can produce bending of the overall observed abundances (filled black circles) measured over the whole line profile. The metallicity derived from the whole line profile could be somewhere between the metallicities of the individual components. If the abundances of enough metals with different refractory and nucleosynthetic properties are observed, the effects of the different processes (a–d) can be separated.