Averaged spectra between 0–10, 10–20, 20–30, 30–40, and 40–50 d in the r- and i-band wavelength ranges. The phases are relative to g-band maximum. Spectra are binned into −3 ≤ x₁ < 0 (dashed) and 0 ≤ x₁ < 3 (solid). The grey shaded regions indicate the spectral regions where a feature appears during the onset of the secondary maximum. The numbers in the brackets represent the number of spectra going into each bin (first number represents the number of spectra in the −3 ≤ x₁ < 0 bin, and the second number are the number of spectra in the 0 ≤ x₁ < 3 bin). We also show the filter response functions of the ZTF r and i bands in black. In the bottom panel we show a spectral timeseries of SN 1999by (a 91bg-like SN Ia).