Danish straits

The Danish straits are the straits connecting the Baltic Sea to the North Sea through the Kattegat and Skagerrak. Historically, the Danish straits were internal waterways of Denmark; however, following territorial losses, Øresund and Fehmarn Belt are now shared with Sweden and Germany, while the Great Belt and the Little Belt have remained Danish territorial waters. The Copenhagen Convention of 1857 made all the Danish straits an international waterway.
Toponymy and geography.
Five straits are named 'belt' (Danish: "bælt"), the only ones in the world. Several other straits are named 'sound' (Danish, Swedish and German: "sund"). Where an island is situated between a "belt" and a "sound", typically the broader strait is called "belt" and the narrower one is the "sound":
Etymology of "sound" / "sund".
The Germanic word "sound" has the same root as the verb "to sunder" in the meaning of "to separate". The Old Norse form of that verb is "sundr". In Norway hundreds of narrow straits separating islands and combining fjords or outer parts of fjords are named "Sund".
Another explanation derives "sound" from an ancient verb "sund" in the meaning of to swim. That way a "sound" is a "swimmable strait". In the Swedish language any strait is called "sund".
The Germanic word "sound" is not related to the Romance languages originated word "sound", which has developed from the Latin "sonus".