Overlay of OH masers with ¹³CO(1–0) emission. The four panels show in grey-scale the position-velocity diagrams along the Galactic longitude axis (integrated over Galactic latitude) for the ¹³CO(1–0) data from the Galactic Ring Survey (Jackson et al. 2006). The symbols in the four panels from top to bottom corresponding OH maser spots from the 1612, 1665, 1667 and 1720 MHz lines, respectively. The colored lines present the spiral arm models from Reid et al. (2016). The missing 1667 MHz masers in the grey-shaded boxes below , from to , and between to , are because this line was not covered in that part of the survey (Beuther et al. 2016).