  The coefficients determining the dilepton decay angular distribution of
vector particles obey certain positivity constraints and a rotation-invariant
identity. These relations are a direct consequence of the covariance properties
of angular momentum eigenstates and are independent of the production
mechanism. The Lam-Tung relation can be derived as a particular case, simply
recognizing that the Drell-Yan dilepton is always produced transversely
polarized with respect to one or more quantization axes. The dilepton angular
distribution continues to be characterized by a frame-independent identity also
when the Lam-Tung relation is violated. Moreover, the violation can be easily
characterized by measuring a one-dimensional distribution depending on one
shape coefficient.
