Left panel: integrated intensity map of the CN (N = 2−1, J = 3/2−1/2, F = 5/2−3/2) hyperfine transition around TW Hya. We also indicate the line of nodes of the disc at a position angle of 240° and the location of the strongest CN emission peak that was used in the polarisation analysis (see text). Right panel: continuum and polarised 226 GHz emission of the TW Hya disc. The colour scale is the total intensity dust emission. The contours, drawn at 3 and 4σ, are the linear polarisation, and the white-line segments indicate the electric vector polarisation angle.