Akershus became a fief in the 16th century. At that time it included the current counties of Hedmark, Oppland, Buskerud and Oslo. It also included municipalities of Askim, Eidsberg and Trøgstad in the county of Østfold. In 1685 Buskerud was separated from Akershus. In 1768 Hedmark and Oppland were also separated from Akershus. In 1842 Christiania (Oslo) was made separate as well. In 1948 Aker was transferred to the county of Oslo.