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 {HEAD ~ at least one polygonal sectional configuration} of {TAIL ~ a grid field} of the first structure 
of
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 { HEAD ~ at least one polygonal sectional configuration } of { TAIL ~ a grid field } of 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 HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH