{TAIL ~ a convex edge}, wherein a trough of the concave edge is aligned with an apex of {HEAD ~ the convex edge}, 
{ TAIL ~ a convex edge } , wherein a trough of the concave edge is aligned with an apex of { HEAD ~ the convex edge } ,
TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH