Multi-wavelength light curve showing the evolution of the flux of VER J0521+211 in different bands and its optical polarisation during the 2020 flare. From top to bottom: VHE gamma rays (with a reference value from MAGIC Collaboration 2020), HE gamma-ray flux, X-ray flux, UV and optical in various bands, R-band flux from various optical telescopes as well as the evolution of PD and EVPA, and radio flux. Shaded background colours define the states A-D used to build the SED of the source, as is described in Table 1.