The concavity has {HEAD ~ a minimum depth} at {TAIL ~ its leading end} and a maximum depth at its trailing end 
at
The concavity has { HEAD ~ a minimum depth } at { TAIL ~ its leading end } and a maximum depth at its trailing end
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH