Any object that cannot be superimposed with its mirror image by translation or rotation in two dimensions has intrinsic 2d chirality, also known as planar chirality. Intrinsic means that the chirality is a property of the object. Any planar pattern that does not have a line of mirror symmetry is 2d-chiral, and examples include flat spirals and letters such as S, G, P. In contrast to 3d-chiral objects, the perceived sense of twist of 2d-chiral patterns is reversed for opposite directions of observation. 2d chirality is associated with circular conversion dichroism, which causes directionally asymmetric transmission (reflection and absorption) of circularly polarized electromagnetic waves.