Subband spectra of Cas A, shown after continuum removal, with the optical depth as a function of frequency. The red shaded regions cover the location of α transitions, orange regions cover β transitions, yellow regions cover γ transitions, green regions cover δ transitions, and blue regions cover ϵ transitions. We note that the widest shaded regions in red cover the spectral lines of the −47 km s⁻¹ and −38 km s⁻¹ Cα components; two recurring thin shaded regions in red represent the weak narrow spectral lines of the 0 km s⁻¹ Cα component and Hα at −47 km s⁻¹.