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