H I longitude-velocity (l−v) diagram constructed by averaging the emission in the latitude range |b|≤ 1.25°. The black contours correspond to the ¹³CO(1–0) l− v at 3, 8, 18, 28, and 38 times the rms noise of the l−v diagram (rms = 0.1 K, T_MB). 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. The magenta box marks the absorption feature could be caused by the Riegel-Crutcher cloud. The ¹³CO(1–0) data are from the Exeter FCRAO CO Galactic Plane Survey (Mottram & Brunt 2010), and the GRS (Jackson et al. 2006).