Spectroscopic sequence from 2500 Å to 10 000 Å and from the time of maximum to t_max+1000 days (rebinned to 5 Å and smoothed with a Savitzky–Golay filter). Spectra up to t_max+100 days (left panel) are characterised by a blackbody continuum with superimposed absorption lines from the SN ejecta, expanding with a velocity of ∼8500 km s⁻¹. Between t_max+100 and t_max+225 days (while SN 2018ibb was behind the sun), the spectroscopic behaviour of SN 2018ibb evolved drastically. The late-time spectra (right panel) are characterised in the blue (< 5000 Å) by a pseudo-continuum and emission lines produced by the interaction of the SN ejecta with circumstellar material and in the red (> 5000 Å) by nebular emission lines from the ⁵⁶Ni-heated SN core. The regions with the fastest evolution are highlighted by the grey-shaded regions. Figure 7 shows the identification of the most prominent features of the photospheric and nebular phases. Regions affected by strong telluric absorption were clipped. Their locations are indicated in Fig. 7.