What is "vingulmark"

Vingulmark (Old Norse Vingulmörk) is the old name for the area in Norway which today makes up the counties of Østfold, western parts of Akershus (excluding Romerike), and eastern parts of Buskerud ( Hurum and Røyken municipalities), and includes the site of Norway's capital, Oslo. During the Middle Ages, Vingulmark was an administrative unit limited to Oslo, Bærum and Asker.