Maryland residents were required to pay both state income tax and county income tax. Maryland residents who earned income in other states could credit any tax paid therein against their Maryland state tax, but not their Maryland county tax. Maryland also imposed income tax on nonresidents who earned income in Maryland. Brian and Karen Wynne (plaintiffs) were Maryland residents who earned income in other states. The Wynnes sued the State of Maryland (defendant), arguing that their inability to credit out-of-state tax paid against the Maryland county tax violated the dormant commerce clause. The United States Supreme Court granted certiorari.