One side or {HEAD ~ both sides} of the panel 20′″ may be covered with {TAIL ~ a layer}, such as an outer covering layer 
One side or { HEAD ~ both sides } of the panel 20′″ may be covered with { TAIL ~ a layer } , such as an outer covering layer
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH