[6] The Øresund Bridge, between the Danish capital Copenhagen and the Swedish city of Malmö, inaugurated on 1 July 2000, connects a bi-national metropolitan area with close to 4 million inhabitants.

Previously, the Ancylus Lake, a fresh-water body occupying the Baltic basin, had been connected to the sea solely via the Great Belt.

Such landforms are common in the area and "ör" is found in the names of many places along the strait, e.g. Helsingør, Skanör, Dragør and Halör, an important center of trade during the Viking Age.

Øresund, like other Danish and Danish-German straits, is at the border between oceanic salt water (which has a salinity of more than 30 PSU or per mille by weight) and the far less salty Baltic Sea.

Well-known examples, for which the bottom salinity makes a distinct breeding border, include lobster, small crabs (Carcinus maenas), several species of flatfish and the lion's mane jellyfish; the latter can sometimes drift into the southwest Baltic Sea, but it cannot reproduce there.

Such events give deep waters in the southern Baltic Sea higher salinity, which makes it possible for cod to breed there.

Denmark maintained military control with the coastal fortress of Kronborg at Elsinore on the west side and Kärnan at Helsingborg on the east, until the eastern shore was ceded to Sweden in 1658, based on the Treaty of Roskilde.

Transitory dues on the use of waterways, roads, bridges and crossings were then an accepted way of taxing which could constitute a great part of a state's income.

To be independent of the Øresund, Sweden carried out two great projects: the foundation of Gothenburg in 1621 and the construction of the Göta Canal from 1810 to 1832.