Schematic images of the two colliding clouds in the top views [(a) and (c)] and the position–velocity diagrams [(b) and (d)]. X-axis and the Z-axis are defined in the same way as in figure 7, and the V-axis corresponds to the radial velocity. (a) and (b) show the RCW 120 case, and (c) and (d) show the S116-S117-S118 case. In (a) and (b), the two clouds can be divided into three sections as follows: A shows the large cloud and the cavity; B shows the interface layer, the small cloud, and the large cloud with the cavity; C shows the large cloud alone. In (c) and (d), the two clouds can be divided into three sections as following: A and B are the same as in panels (a) and (b), and C shows the interface layer alone. The difference between the two models is whether the interface layer is inside or outside of the large cloud. In (d), there is no gas, shown by the dashed line. In section A, the majority of the clouds before the collision have been scrapped by the collision and are decreasing for the two models. In section B, the X–V diagrams show a “V”-shaped structure for the two models, although the right-hand side of the “V” in (b) is denser than that in (d). In section C, there is only gas of the large cloud in panels (a) and (b), whereas there is only gas of the interface layer in (c) and it appears vertical in the X–V diagram in (d).