The Saxon duchy began fragmenting in the 15th century, as a result of the old German succession law that divided inheritances among all sons. In addition, every son of a Saxon duke inherited the title of duke. Brothers sometimes ruled the territory inherited from their father jointly, but sometimes they split it up. Some of the Ernestine duchies retained their separate existence until 1918. Similar events in the houses of Reuss and Schwarzburg led to all of Thuringia becoming a tangle of small states from the late 15th century until the early 20th century.