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