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