Flux spectrum, polarisation degree P, and normalised Stokes Q and U parameters of SN 2018bsz at 10.2 d (left) and 38.4 d (right). The polarisation and Stokes spectra have been binned to 25 Å but the flux spectra are unbinned. Vertical lines show the location of major spectroscopic features. Note that at 10.2 d hydrogen lines are shown at blueshift of −8000 km s⁻¹. The dash-dotted lines in the right panels present the estimation for interstellar polarisation (ISP) in our case B (ISP B), where it has been assumed that the strongest emission lines completely depolarise the spectrum (see Sect. 5.1). The data have not been corrected for the ISP.