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