Position–velocity diagrams for HCO⁺ (top) and N₂H⁺ (bottom) emission along the DR21 ridge. The offset is measured from south to north (see Fig. 4 for the exact location of the cut). The magenta dashed contour marks the H41 α emission from the Η II region while the orange dashed lines mark the source velocity (−3 km s⁻¹). For the N₂H⁺ line, the velocities of the three resolved hyperfine structure components are marked at 4, −3, and −10 km s⁻¹, respectively.