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