Illustration of linewidth broadening caused by beam averaging. Left panel: same as the left panel of Fig. 1, but showing only the positions and estimated widths of the Perseus (Per) and Outer (Out) spiral arms. Black solid lines show curves of constant projected v_LSR values. The red line shows a random line of sight with the corresponding intersections with the v_LSR = 20 km s⁻¹ curve indicated with red dots. The centre panels illustrate the change in spatial extent of the beam (black circle) for a region with two blended velocity components embedded at the near (bottom centre) and far (top centre) distance. The right panels illustrate the resulting observed spectra (black line) and Gaussian fit components (blue lines).