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