Sketch of the method used to estimate the effect of a nonzero vertical emitting width for CN. Panel (A) shows the geometry of CN emission in IM Lup, considering an inclination of 47.8° with respect to the observer. For the vertical structure, the solid curve indicates the best-fit vertical profile of CN emission and the dashed curve the extent considering a fixed vertical width. In each case, the blue dot is the observed parcel of gas at position (r₁, z₁) that has to cross the line-of-sight path as indicated by the red line, leaving the zone of CN emission at the red dot location (r₂, z₂) For different radial distances, the path varies. Panel (B) shows the velocity variation between the blue dot and the final red dot positions. Each solid-line Gaussian is centered at a determined velocity, depending on their (r, z) position along the line-of-sight path. The measured linewidth (ΔVmeasured) is taken from the resulting average Gaussian, shown as a dashed black curve and compared to the intrinsic value (ΔVgas).