Longitude-resolved mean and variability of the polarised emission from PSR B1919+21 at 1414 MHz. Panel a: both the ellipticity angle of the average Stokes parameters (points with error bars) and a histogram of the ellipticity angles of the individual pulses (greyscale intensity). Similarly, panel b: both the position angle of the average Stokes parameters and a histogram of the position angles of the individual pulses. Panel c: integrated profile in total intensity (black, upper solid), linear (red, dashed) and circular polarisation (blue, dot-dashed). These profiles were obtained by averaging together all the available data. In panel d, the black solid line shows the standard deviation of the total intensity, and the red dashed, green dot-dashed, and blue dotted lines display the square roots of the eigenvalues (from largest to smallest, respectively) of the 3 × 3 partition of the covariance matrix that describes the fluctuations of the polarisation vector S = (Q, U, V).