By “congruent” is meant in this context that a polygonal sectional configuration, parallel to the outer skin, of a grid field of the second grid structure fits onto at least one polygonal sectional configuration of {HEAD ~ a grid field} of {TAIL ~ the first structure} 
By “ congruent ” is meant in this context that a polygonal sectional configuration , parallel to the outer skin , of a grid field of the second grid structure fits onto at least one polygonal sectional configuration of { HEAD ~ a grid field } of { TAIL ~ the first structure }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL