Öresund (Swedish spelling) or Øresund (Danish spelling), or sometimes The Sound in English, is the strait that separates Zealand from Scania, and thereby Denmark from Sweden. Öresund connects the Baltic Sea to the Atlantic Ocean via the bay of Kattegat, and is one of the busiest waterways in the world.
Political control of Öresund has been an important issue in Danish (and Swedish) history. Danish military control was maintained with the coastal fortress of Kronborg at Elsinore (Danish: Helsingør) on the western side, and Kärnan at Helsingborg on the eastern side, of Öresund's neck, where the strait is only 4.5 kilometers wide.
In 1429 King Eric of Pomerania introduced the Øresund-Toll. Every ship passing Elsinore had to pay duty to the Danish Crown (regardless if the cargo was en route to or from Denmark or not). The Øresund-Toll was for centuries the Crown's most important income, making the kings relatively independent of Denmark's Privy Council and aristocracy. After 1658, when Terra Scania was ceded to Sweden, the toll could not be enforced as before, but it wasn't abolished until 1857.