{HEAD ~ A gap} is defined between {TAIL ~ each lobe}, the gap being defined at the radial outer edge 
is defined between
{ HEAD ~ A gap } is defined between { TAIL ~ each lobe } , the gap being defined at the radial outer edge
HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH