How wide is the location of the Battle of the Sound at its narrowest point?
Battle of the Sound: The naval Battle of the Sound took place on 8 November 1658 (29 October O.S.) during the Second Northern War, near the Sound or Øresund, just north of the Danish capital, Copenhagen. Øresund: Øresund or Öresund (Danish: "Øresund" , ] ; Swedish: "Öresund" , ] ), commonly known in English as the Sound, is a strait which forms the Danish–Swedish border, separating Sjælland (Denmark) from Skåne (Sweden). Øresund:  It is 4 km wide at its narrowest point between Helsingør in Denmark and Helsingborg in Sweden.
4 km