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