P₃-folded polarisation state of the first 500 pulses in the PSR B1919+21 dataset. From top to bottom are the total intensity; the position angle, θ = tan⁻¹(U′/Q′)/2; the fractional linear polarisation, ; the ellipticity angle, ϵ = sin⁻¹(V′/L′)/2; and the fractional circular polarisation, ; where P₃-folded quantities are shown with primed symbols (e.g., Q′ and U′) and the symbols for single-pulse quantities are not primed (e.g., Q and U). That is, and are derived from the single-pulse Stokes parameters before P₃ folding and θ and ϵ are derived from the P₃-folded Stokes parameters. Furthermore, and are integrated only when the single-pulse total intensity exceeds 4 times the standard deviation of the of noise. In each panel, the modulation period (P₃ ≃ 4.2 P₁) is plotted twice to better illustrate the continuity of the pattern.