The curtain or {HEAD ~ barrier} is formed by {TAIL ~ a region} of flowing conditioned air directed across the opening 
is formed by
The curtain or { HEAD ~ barrier } is formed by { TAIL ~ a region } of flowing conditioned air directed across the opening
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH