Halfdan the Black was a king of Vestfold. He belonged to the House of Yngling and was the father of Harald Fairhair, the first king of a unified Norway.

Vestfold is a county and a current electoral district in Eastern Norway. Located on the western shore of the Oslofjord, it borders Buskerud and Telemark counties. The county administration is located in Tønsberg, Norway's oldest city, and the largest city is Sandefjord. With the exception of the city-county of Oslo, Vestfold is the smallest county in Norway by area. Vestfold was until 2019 the only county in which all municipalities had declared Bokmål to be their sole official written form of the Norwegian language.