In 1255 the Duchy of Bavaria was partitioned between the brothers Louis II and Henry I, who had been jointly ruling the Duchy since the death of their father in 1253.
Louis II, the elder brother, took UPPER BAVARIA (OBERBAYERN), which swept in a great arc from southwestern Bavaria to the north, and included the upper lands, within Bavaria, on the Rivers Inn, Isar and Danube and also the lands of the Nordgau, drained by the Naab, a northern tributary of the Danube. He was also Count Palatine of the Rhine. When he died in 1294 he left two sons, who, when they had both come of age, ruled jointly for a while before partitioning their territories in 1310. The elder, the Count Palatine Rudolf, held Munich in the south and Burglengenfeld in the Nordgau; the younger, Louis IV, ruled in Ingolstadt on the Danube and in Amberg in the Nordgau. The brothers quarrelled. Louis was elected German King in 1314, but Rudolf supported his Habsburg cousin rather than his brother. Rudolf was driven out in 1317 but in 1329 Louis reached an accommodation with his nephews, whereby they held the Palatinate lands on the Rhine and Neckar together with northern Upper Bavaria, that is, the old Nordgau, which was thenceforward called the Upper Palatinate (Oberpfalz) and still is. Most of the Oberpfalz returned to Bavaria in 1628, but it was only from 1777 that some of its lands again shared a common ruler with Bavaria. From 1329 therefore Upper Bavaria does not include the Upper Palatinate.
The Emperor Louis IV acquired Lower Bavaria in 1340 when Henry’s male descent was extinguished. After his death in 1347 his six sons governed Bavaria jointly, but in 1349 they partitioned the Duchy, revising the partition in 1351. In that year Louis V, the eldest son of the Emperor, who had been ruling Brandenburg (or trying to), returned to Bavaria and took Upper Bavaria as his share. With him were associated two of his half-brothers, who were endeavouring to rule in Brandenburg as Louis V’s successors. He died in 1361, followed by his son in 1363, whereupon his full brother, Stephen II, added Upper Bavaria to his own share of the greater part of Lower Bavaria, ignoring the rights of his two half-brothers in Brandenburg.. Stephen II died in 1375 and eventually in 1392 his three sons partitioned the inheritance, the eldest and third sons holding Upper Bavaria in two Duchies, those of Ingolstadt in the north and Munich in the south.
The Upper Bavarian Dukes acquired a share of Lower Bavaria after the Straubing line had died out in 1425, but in 1445 the lands of Bavaria-Ingolstadt were added to the remaining Lower Bavarian Duchy, that of Landshut. Eventually Bavaria was reunited in 1505, though another partiiton meant that complete union did not come until 1545.
In present-day Bavaria OBERBAYERN is an administrative region (Regierungsbezirk) which extends northwards from the Alps to the Danube at Ingolstadt, and beyond to include the town of Eichstätt and some of the valley of the River Altmühl. Its eastern boundary is the River Salzach; the western boundary is in the vicinity of the River Lech, sometimes west of it, sometimes east, only briefly on it. The land around Eichstätt was once reckoned to be in Franconia; the valley of the River Paar is in the administrative region of Schwaben, and so the old castle of Wittelsbach, which gave its name to the long-lasting Bavarian dynasty, today lies outside Oberbayern.