Also 2d chirality can arise from the relative arrangement of different (achiral) components. In particular, oblique illumination of any planar periodic structure will result in extrinsic 2d chirality, except for the special cases where the plane of incidence is either parallel or perpendicular to a line of mirror symmetry of the structure. Strong circular conversion dichroism due to extrinsic 2d chirality has been observed in metamaterials.