Braunschweig (German pronunciation), also called Brunswick in English, is a city of 252,768 people (as of 31 December 2015), located in the state of Lower Saxony, Germany. It is situated north of the Harz mountains at the furthest navigable point of the Oker river, which connects to the North Sea via the rivers Aller and Weser. A powerful and influential centre of commerce in medieval Germany, Braunschweig was a member of the Hanseatic League from the 13th until the 17th century. It was the capital city of three successive states: the Principality of Brunswick-Wolfenbüttel (1269–1432, 1754–1807 and 1813–1814), the Duchy of Brunswick (1814–1918) and the Free State of Brunswick (1918–1946). Today, Braunschweig is the second largest city in Lower Saxony and a major centre of scientific research and development.