All foreign ships passing through the strait, whether
en route to or from Denmark or not, had to stop in
Helsingør and pay a toll to the Danish Crown. If a ship refused to stop, cannons in both Helsingør and
Helsingborg could open fire and sink it. In 1567, the toll was changed into a 1–2% tax on the
cargo value, providing three times more revenue. To keep the captains from understating the value of the cargo on which the tax was computed, the right to purchase the cargo at the stated value was reserved. In order to avoid ships simply taking a different route, tolls were also collected at the two other Danish straits, the
Great Belt and the
Little Belt; sometimes non-Danish vessels were forbidden to use any other waterways but the
Øresund, and transgressing vessels were confiscated or sunk. The Sound Dues remained the most important source of income for the Danish Crown for several centuries, thus making Danish kings relatively independent of Denmark's
Privy Council and
aristocracy. However, the dues were an irritant to nations engaged in trade in the
Baltic Sea, especially
Sweden. Sweden had initially been exempted from the dues at the time of their introduction because it was then in the
Kalmar Union along with Denmark. However, after the
Kalmar War and the
Treaty of Knäred in 1613
Denmark-Norway introduced dues on cargoes from Sweden's Baltic possessions and on non-Swedish ships carrying Swedish cargo. The friction over the Dues was an official
casus belli (reason for war) of the
Torstenson War in 1643. In 1645, at the
Treaty of Brömsebro, Denmark-Norway had to cede the provinces
Jämtland and
Härjedalen as well as the Baltic Sea islands of
Gotland and
Saaremaa (Ösel) to Sweden as a consequence of the Torstenson War. Swedish shipping also became exempt from the Sound Dues by the terms of this treaty. The exemption was withdrawn after Sweden's defeat in the
Great Northern War and the
Treaty of Frederiksborg of 1720, although the eastern shore of the Sound was now Swedish (since the
Treaty of Roskilde in 1658).
Abolishment The Copenhagen Convention, which came into force on 14 March 1857, abolished the dues and all
Danish straits were made
international waterways free to all commercial shipping. ==Statistics==