In medieval times the territory was ruled by the counts of Henneberg and the princes of Schwarzburg, two local dynasties. While Ilmenau was ruled by Henneberg, Arnstadt belonged to Schwarzburg. When the Henneberg family became extinct in 1583, Ilmenau was acquired by Saxony. The principality of Schwarzburg was split up between family members into several tiny states. One of those states (Schwarzburg-Arnstadt) was based in Arnstadt; it was founded in 1574 and existed until 1716, when it became part of Schwarzburg-Sondershausen. It became part of Thuringia when the Freestate of Thuringia was founded in 1920.