Linear polarisation properties of PSR B1821−24A as observed with GBT in L-band (BPDR). Grey curves refer to the vertical axes on the right-hand side. The bottom two panels zoom into the double notches at φ ≈ 100°. a) Total flux I (black solid) and the linear polarisation fraction Π = L/I (grey). b) The PA (black) and log I (grey). c) The total flux S/N (black solid) and the polarisation fraction (grey). d) The PA (black) and log I (grey). The straight line presents the PA variations anticipated in the absence of the notches. There is a drop in Π and a change in PA at the notches. The zero point of PA (whether observed or modelled) is arbitrary in all figures of this paper.