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