Location of SN 2018bsz on the Q – U plane at 10.2 d (left) and 38.4 d (right). The individual points are coloured according to their wavelength as indicated in the colour bar. Thin dashed lines have been drawn at Q = 0, U = 0 and P = 1 to guide the eye. The two ellipses in red are the outcome of a principal component analysis (Maund et al. 2010) where the major axis of the ellipse is aligned with the direction of the maximum variance of the data and the axial ratio b/a parameterises the ratio of polarisation carried by the orthogonal to the dominant direction (minor to major axis). The locations of two alternative ISP solutions that have been examined (ISP A and ISP B; see Sect. 5.1) have been marked with grey hexagons, but the data have not been corrected for any ISP. There is a large change in the SN polarisation between the two epochs shown the large shift in the barycentre (, ), which is independent of the ISP. Such large changes on the Q – U plane have rarely been observed for SN explosions.