In general, a vibrational mode transforming as Λ will couple to an electronic state transforming as Γ if the symmetric part of the Kronecker product [Γ ⊗ Γ] S contains Λ, unless Γ is a double group representation when the antisymmetric part {Γ ⊗ Γ} A is considered instead. Modes which do couple are said to be JT-active.