A ledge or {HEAD ~ mandible engaging support surface} is defined between the upper tier and {TAIL ~ lower tier} and terminates in an exterior opening 
is defined between
A ledge or { HEAD ~ mandible engaging support surface } is defined between the upper tier and { TAIL ~ lower tier } and terminates in an exterior opening
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH