When a court announces a new rule, whether it applies to past conduct and to pending cases is a separate question with its own doctrine.
Civil cases. A rule applied to the parties before the court generally applies to all cases open on direct review, on the principle that courts declare rather than make law. Selective prospectivity — applying a rule to the parties but not to others — has been rejected in federal practice.
Cases already final are generally unaffected, and reopening them requires an independent ground for relief from judgment.
State practice varies, and several states retain a balancing approach considering whether the decision established a new principle, whether retroactive application would further or retard its operation, and the inequity of retroactive application including reliance.
Statutory changes are presumed prospective unless the legislature clearly states otherwise, and a statute is retroactive if it would impair rights a party possessed when acting or attach new consequences to completed transactions.
Practical consequence. A favourable decision issued while an appeal is pending should be brought to the court’s attention by supplemental authority immediately. A decision issued after judgment is final is generally of no help except through the narrow relief-from-judgment routes.