Synthetic longitude-velocity diagram of the ¹²CO(1–0) emission line constructed using our Galactic model by placing the observer at position P1 (see Fig. 2). The velocities are averaged within a latitude of |b| < 1.2°. Purple dots and vertical lines show detections of spiral arm tangent points, as determined from CO. Yellow dots represent observations of maser emissions coming from high-mass star-forming regions within our own Milky Way presented in Reid et al. (2019). In general, the detections depend on the tracers used (see Table 1). The detections are labeled A - J with increasing Galactic longitude. The magnitude of the CO intensity is normalized by its peak value.