Mrkunlex
The Duchy of Saxony (Low German: Hartogdom
Sassen, German: Herzogtum Sachsen) was originally the area settled by the Saxons in the late Early Middle Ages, when they were subdued by Charlemagne during the Saxon Wars from 772 and incorporated into the Carolingian Empire (Francia) by 804. Upon the 843 Treaty of Verdun, Saxony was one of the five German stem duchies of East Francia; Duke Henry the Fowler was elected German king in 919.
Upon the deposition of the Welf duke Henry the Lion in 1180, the ducal title fell to the House of Ascania, while numerous territories split from Saxony, such as the Principality of Anhalt in 1218 and the Welf Duchy of Brunswick-Lüneburg in 1235. In 1296 the remaining lands were divided between the Ascanian dukes of Saxe-Lauenburg and Saxe-Wittenberg, the latter obtaining the title of Electors of Saxony by the Golden Bull of 1356.
Geography
The Saxon stem duchy covered the greater part of present-day Northern Germany, including the modern German states (Länder) of Lower Saxony and Saxony-Anhalt up to the Elbe and Saale rivers in the east, the city-states of Bremen and Hamburg, as well as the Westphalian part of North Rhine-Westphalia and the Holstein region (Nordalbingia) of Schleswig-Holstein.
In the late 12th century, Duke Henry the Lion also occupied the adjacent area of Mecklenburg (the former Billung March).
Source:
https://en.m.wikipedia.org/wiki/Duchy_of_Saxony
