Diffuse Interstellar Bands: How Are They Related to Known Gas-Phase
  Constituents of the ISM?
  In this brief review of recent work relating the DIBs to other gas-phase
constituents of the ISM, we explore correlations between DIB equivalent widths
and the column densities of various atomic and molecular species, drawn from a
large database constructed for that purpose. The tightness and slopes of the
correlations can provide information on how the DIBs might be related to those
species (physically, chemically, spatially) and on various properties of the
DIB carriers. Deviations from the mean relationships can reveal dependences of
DIB strengths on other parameters, regional variations in DIB behavior, and
individual sight lines where unusual environmental conditions affect the DIBs.
Variations in DIB profiles (e.g., wings, substructure) and relative strengths
may be related to differences in physical conditions inferred from atomic
and/or molecular absorption lines.
