{HEAD ~ the extension upper surface} is flush with {TAIL ~ the upper surface} when the extension member is at the first position 
{ HEAD ~ the extension upper surface } is flush with { TAIL ~ the upper surface } when the extension member is at the first position
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH