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Several rotational invariant quantities for the lepton angular distributionsin Drell-Yan and quarkonium production were derived several years ago, allowingthe comparison between different experiments adopting different referenceframes. Using an intuitive picture for describing the lepton angulardistribution in these processes, we show how the rotational invariance of thesequantities can be readily obtained. This approach can also be used to determinethe rotational invariance or non-invariance of various quantities specifyingthe amount of violation for the Lam-Tung relation. While the violation of theLam-Tung relation is often expressed by frame-dependent quantities, we notethat alternative frame-independent quantities are preferred.

On the Rotational Invariance and Non-Invariance of Lepton Angular Distributions in Drell-Yan and Quarkonium Production