Spiral arms in the fourth quadrant are represented in a V_LSR-longitude (V-l) plot, adapted from Vallée (2008): red: Norma-Cygnus; blue: Scutum-Crux; green: Sagittarius-Carina; magenta: Perseus. The rectangular boxes indicate the longitude extent in the Galactic plane at a latitude b = 0° of the HIFI [C ii] spectral line map data presented here. Note that the maps cover the tangencies of the Norma, Crux, and start of the Perseus arms. The vertical dashed line represents a line of sight at a given longitude that intercepts multiple spiral arms, thus demonstrating the need for velocity-resolved spectral line data to separate them.