In the general case, the constitutive relations for bi-anisotropic materials read D = ε ε E + ξ ξ H , B = ζ ζ E + μ μ H , where ε ε and μ μ are the permittivity and the permeability tensors, respectively, whereas ξ ξ and ζ ζ are the two magneto-electric tensors. If the medium is reciprocal, permittivity and permeability are symmetric tensors, and ξ ξ = − − ζ ζ T = − − i κ κ T , where κ κ is the chiral tensor describing chiral electromagnetic and reciprocal magneto-electric response. The chiral tensor can be expressed as κ κ = 1 3 tr ⁡ ⁡ ( κ κ ) I + N + J , where tr ⁡ ⁡ ( κ κ ) is the trace of κ κ , I is the identity matrix, N is a symmetric trace-free tensor, and J is an antisymmetric tensor. Such decomposition allows us to classify the reciprocal bianisotropic response and we can identify the following three main classes: (i) chiral media ( tr ⁡ ⁡ ( κ κ ) ≠ ≠ 0 , N ≠ ≠ 0 , J = 0 ), (ii) pseudochiral media ( tr ⁡ ⁡ ( κ κ ) = 0 , N ≠ ≠ 0 , J = 0 ), (iii) omega media ( tr ⁡ ⁡ ( κ κ ) = 0 , N = 0 , J ≠ ≠ 0 ).