Comptroller of the State of Maryland v. Wynne

Maryland residents paid both a state income tax (creditable for taxes paid to other states) and a separate county income tax (not creditable), while Maryland also taxed nonresidents' Maryland-source income; the Wynnes (plaintiffs), Maryland residents earning income in other states, could not credit their out-of-state taxes against the county tax and sued the Comptroller (defendant), arguing the scheme violated the dormant Commerce Clause. The Supreme Court granted certiorari.