Their symmetry group , the dihedral group of order 4 , has eight elements .
2 nonominoes have four axes of reflection symmetry , aligned with the gridlines and the diagonals , and rotational symmetry of order 4 .
nonominoes ' symmetry group , the dihedral group of order 4 , has eight elements .
2 nonominoes have four axes of reflection symmetry , aligned with the gridlines and the diagonals , and rotational symmetry of order 4 .