Top panel: ladder plot for model r0e2 at 32 d after explosion (which corresponds to the time of maximum), covering the UV and optical ranges (this model yields a good match to Gaia16apd at bolometric maximum; see Sect. 10.3). The upper part shows the full spectrum F_λ together with the continuum (thin line). Line emission contributes throughout the optical and causes the excess flux above the continuum level. There are no true “continuum” windows – the total model flux is 50% greater everywhere except in two P-Cygni troughs. This excess line flux impacts the interpretation of a color temperature based on a blackbody fit. In the UV, lines cause a strong blanketing, with the total flux depleted below the continuum level. The lower part shows the ratio of the full spectrum (F_λ, All) and the spectrum computed by omitting the bound-bound transitions of a given ion (F_λ, less). Bottom panel: optical spectrum (normalized to the maximum flux in the range 2500 − 10 000 Å) of model r0e2 at 240.0 d after explosion. Vertical marks at the top correspond to the air wavelength of lines with a Sobolev equivalent width (EW) greater than 7000 Å (the ions shown at top are displayed in the order of appearance of transitions, starting at the shortest wavelength displayed). Here, 251 lines with an EW between 7000 and 49 340 Å are shown. The thickness of each bar scales linearly with the line EW. The name of the corresponding ion lies at the left of each series of marks.