The walls can be formed so that {HEAD ~ the bottom ply wall} is received within {TAIL ~ the channel} defined by the top ply wall, or so that the top and bottom ply walls are concentric with each other 
is received within
The walls can be formed so that { HEAD ~ the bottom ply wall } is received within { TAIL ~ the channel } defined by the top ply wall , or so that the top and bottom ply walls are concentric with each other
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH