The subject here is the absorption coefficient, expressing the net power loss from the field over a unit path. At its heart is the line shape, which may be identified with the power spectral density function for fluctuations of the active dipole in the presence of an equilibrium bath of perturbers, and, as such, should satisfy the fluctuation–dissipation theorem. The more general properties of the absorption coefficient, which must reflect this balance, are first examined in some detail, particularly for the Van Vleck–Huber form. It is then shown that this, when expanded as a sum over individual lines, may be folded into more compact expressions. Outside the line core, these expressions must incorporate the fluctuation–dissipation theorem, and special attention is given to distinguish this case and that of the core itself, where it is of no consequence. Even the very general Fano theory does not, as it stands, satisfy the theorem, and can be used for the far-wing line shape only if these expressions are modified. Finally, some account is given of how they may be used with a molecular line database, and how a calculation of radiative transfer might proceed in the simplest of cases.