Evolution of the Hα profile of AT 2021biy. The Hα lines are shown in velocity space, with zero velocity marked by a red dotted line. Phases (red colour) and resolution (blue colour) of each spectrum are labeled on the right-hand side. Phases are relative to r-band maximum. All of the spectra are corrected for redshift and reddening. Note that some low-S/N spectra are not shown in this figure.