Ernestine duchies
Albert, grandson of  Albert "the Bear" inherited the Duchy of Saxony from his father.   Albert's sons divided Saxony into the duchies of Saxe-Lauenburg and Saxe-Wittenberg.  When duke of Saxe-Wittenberg died without an heir in 1422, the Emperor Sigismund gave the duchy to Frederick IV of the House of Wettin.  Frederick was already Margrave of Meißen and Landgrave of Thuringia, and then became Frederick I, Elector of Saxony.  Frederick was followed as Elector by his son Frederick II, and when Frederick II died in 1464 his sons Ernest and Albert took over.  Because he was the older, Ernest became Elector of Saxony, but the brothers shared ruling the lands until 1485.  Then Ernest took northern Meißen, southern Thuringia, and Wittenberg, with Albert receiving northern Thuringia and southern Meißen