When {TAIL ~ the bottom ply} includes {HEAD ~ two walls} , the top and bottom ply walls are offset from each other so that they mesh with each other 
When { TAIL ~ the bottom ply } includes { HEAD ~ two walls } , the top and bottom ply walls are offset from each other so that they mesh with each other
OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH