Diagonal symmetry (2 diagonals)
A polyomino has reflective symmetry over both axes if it is identical to its reflection over both diagonals. A polyomino with this type of symmetry naturally has reflective symmetry over a diagonal. It also has two-fold rotational symmetry, since a 180˚ rotation is equivalent to flipping over one axis and then another. It is represented by the Klein four group.
The smallest polyomino with this symmetry is the heptomino below. This symmetry is possible for every size except for nonominoes ().