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