Optically thin HCN J = 1–0 (~88.6 GHz), 2–1 (~177.3 GHz) and 3–2 (~265.9 GHz) spectra for a cloud with Δv ≃ 1 km s⁻¹. Each panel shows the velocity separation from the strongest HFS component. As line opacities increase, line overlaps in the J = 2–1 and 3–2 HFS transitions affect the global excitation of HFS levels. Red, green, and cyan arrows show specific lines that overlap and produce most of the anomalous HFS emission discussed in the text (see also Fig. A.1).