{HEAD ~ The first position} is used by {TAIL ~ apparatus} 10 when a background or reference reading is desired 
{ HEAD ~ The first position } is used by { TAIL ~ apparatus } 10 when a background or reference reading is desired
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH