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