Copenhagen Convention of 1857

The Sound Dues (or the Sound Toll; Danish: Øresundstolden) were a toll on the use of the Øresund which constituted up to two thirds of Denmark's state income in the 16th and 17th centuries. The dues were introduced by King Eric of Pomerania in 1429 and remained in effect until the Copenhagen Convention of 1857. (With the only exception of Swedish ships between 1660 and 1712)

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 of 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 elegant solution was chosen to reserve the right to purchase the cargo at the value stated.

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 . 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 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 of the Torstenson War in 1643.

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