What is "revierderby"

The Revierderby , also known as the Ruhr derby , is the derby in German football between Borussia Dortmund and Schalke 04. It is named after the Ruhr region (also known in German as Revier), where both clubs are located. Also as Revierderby or small Revierderby (kleines Revierderby) are local derbies know between other teams from the Ruhr region.