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