Ratio map l−v diagram of H I over ¹³CO (T_B (H I)/T_B(¹³CO)) averaged over galactic latitude. The ¹³CO(1–0) data do not cover the velocity range v_LSR < −5 km s⁻¹. The overlaid curves trace the Sagittarius, Scutum, Norma, Perseus, Outer, and Outer Scutum-Centaurus (OSC) arms, and smaller features (Local Spur, Aquila Spur, and Aquila Rift) taken from Reid et al. (2016). The near and far sides of the arms are plotted with dashed and solid lines, respectively.