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