{TAIL ~ the leading edge} and {HEAD ~ the trailing edge} define a first gap having a first gap width; and 
{ TAIL ~ the leading edge } and { HEAD ~ the trailing edge } define a first gap having a first gap width ; and
TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH