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