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There is little consistency in the use of "sound" in English-language place names.
A sound is often formed by the seas flooding a river valley. This produces a long inlet where the sloping valley hillsides descend to sea-level and continue beneath the water to form a sloping sea floor. The Marlborough Sounds in New Zealand are good examples of this type of formation.
Sometimes a sound is produced by a glacier carving out a valley on a coast then receding, or the sea invading a glacier valley. The glacier produces a sound that often has steep, near vertical sides that extend deep under water. The sea floor is often flat and deeper at the landward end than the seaward end, due to glacial moraine deposits. This type of sound is more properly termed a fjord (or fiord). The sounds in Fiordland, New Zealand, have been formed this way.
A sound generally connotes a protected anchorage. It can be part of most large islands.
In the more general northern European usage, a sound is a strait or the narrowest part of a strait. In Scandinavia and around the Baltic Sea, there are more than a hundred straits named Sund, mostly named for the island they separate from the continent or a larger island.
In contrast, the Sound is the common international short name for Øresund, the narrow stretch of water that separates Denmark and Sweden, and is the main waterway between the Baltic Sea and the North Sea. It is also a colloquial short name, among others, for Plymouth Sound, England.
In areas explored by the British in the late 18th Century, particularly the northwest coast of North America, the term "sound" was applied to inlets containing large islands, such as Howe Sound in Vancouver and Puget Sound in Washington State. It was also applied to bodies of open water not fully open to the ocean, such as Caamaño Sound or Queen Charlotte Sound in Canada, or broadenings or mergings at the openings of inlets, like Cross Sound in Alaska and Fitz Hugh Sound in British Columbia.
In the United States, Long Island Sound separates Long Island from the eastern shores of the Bronx, Westchester County, and southern Connecticut, but on the Atlantic Ocean side of Long Island, the body of water between Long Island and its barrier beaches is termed the Great South Bay. Pamlico Sound is a similar lagoon that lies between North Carolina and its barrier beaches, the Outer Banks, in a similar situation. The Mississippi Sound separates the Gulf of Mexico from the mainland, along much of the gulf coasts of Alabama and Mississippi. On the West Coast, Puget Sound, by contrast, is a deep arm of the ocean.
The word sund is already documented in Old Norse and Old English as meaning "gap" (or "narrow access"). This suggests a relation to verbs meaning "to separate", such as absondern and aussondern (German), söndra (Swedish), sondre (Norwegian), as well as the English noun sin, German Sünde ("apart from God's law"), and Swedish synd. English has also the adjective "asunder" and the noun "sundry', and Swedish has the adjective sönder ("broken").
In Swedish and in both Norwegian languages, "sund" is the general term for any strait. In Swedish and Nynorsk, it is even part of names worldwide, such as in Swedish "Berings sund" and "Gibraltar sund", and in Nynorsk "Beringsundet" and "Gibraltarsundet".