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