{HEAD ~ A double wall} is a wall which defines a channel in {TAIL ~ the ply} in which it is formed 
{ HEAD ~ A double wall } is a wall which defines a channel in { TAIL ~ the ply } in which it is formed
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH