{HEAD ~ An engagement} occurs between {TAIL ~ the first and second elements} in the closed condition of the abutment structure 
{ HEAD ~ An engagement } occurs between { TAIL ~ the first and second elements } in the closed condition of the abutment structure
HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH