The transition moment integral is zero if the transition moment function, ψ ψ 1 ∗ ∗ μ μ ψ ψ 2 , is anti-symmetric or odd, i.e. y ( x ) = − − y ( − − x ) holds. The symmetry of the transition moment function is the direct product of the parities of its three components. The symmetry characteristics of each component can be obtained from standard character tables. Rules for obtaining the symmetries of a direct product can be found in texts on character tables.