Illustrative line-broadening models for the C iv 5801 Å line. Upper panels: results in the Fourier domain. The solid black lines show the transform of the observed profile (i.e.. the transform of the black spectrum in the lower panels); the red lines show the transforms of synthetic profiles computed with isotropic Gaussian macroturbulence (with dispersion σ, in km s⁻¹), rotational broadening (with v_e sin i=v), and Gaussian noise; and the green lines show the transforms for rotational broadening alone. Any model with v_e sin i≲ 60 km s⁻¹ (and appropriate macroturbulence) is acceptable. Lower panels: results in the wavelength domain. The observed spectrum is shown in each panel in green (the directly observed profile) and black (the version of the profile used in the analysis, after re-rectification to take out the diffuse interstellar band). The synthetic profiles, shown in red, are the inverse transforms of the models shown in red in the upper panels. At this scale, the synthetic profiles (which have been modestly scaled in intensity to match the observed line strength) are almost indistinguishable from each other, and from the observed spectrum, in the wavelength domain.