Grand-design spiral galaxy NGC 628. The background image is the 21 cm map from The HI Nearby Galaxy Survey (THINGS, Walter et al. 2008). Red ellipses denote HI supershells from Bagetakos et al. (2011). Blue plus-signs denote the 650 HII regions identified by Honig & Reid (2015). Their spiral arm designations, A and B, are also indicated. The large green circle indicates the median co-rotation radius of 4.6 ± 1.2 kpc from a number of studies as compiled by Scarano & Lépine (2013). For the first half-turn, arm A has no HI supershell on its trailing edge, but four are close to or even overlap the leading edge in the way envisaged by our model. Arm B begins just inside of co-rotation and has three prominent HI supershells at its leading edge, with only a minor one towards the trailing edge. From about the co-rotation radius outwards, HI supershells are no longer at the edges of the HII arm, but appear all over it.