Upper Saxony
It was particularly to distinguish the lands from 'Lower Saxony', a concept which arose later in popular usage (though never enforced) as a term for the original Saxon lands in north and west Germany (where Low German dialects had spread), in what is now the state of Lower Saxony, as well as the adjacent Westphalian region, Holstein and the western part of today's Saxony-Anhalt north of the Harz range (Eastphalia).
Old Saxony
For the most part, the Saxon lands were a broad plain, save on the south, where they rose into hills and the low mountainous country of the Harz and Hesse. This low divide was all that separated the country of the Saxons from their ancient enemies and ultimate conquerors, the Franks. The lack of clear physical definition along this border, from time immemorial, had been the cause of incessant tribal conflict between them. Saxons as inhabitants of present-day Northern Germany are mentioned in 555, when Theudebald, the Frankish king, died and the Saxons used this opportunity for war. The Saxons were defeated by Chlothar I, Theudebald's successor. Some of their Frankish successors fought against the Saxons, while others were allied with them; Chlothar II won a decisive victory against the Saxons.