The outer perimeter includes {HEAD ~ a front perimeter edge} and {TAIL ~ a rear perimeter edge} that are joined to each other along a seating axis 
The outer perimeter includes { HEAD ~ a front perimeter edge } and { TAIL ~ a rear perimeter edge } that are joined to each other along a seating axis
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH