The Viking colony of the Seine, largely Danish, had Rolf (Rollo/Rollon) as leader from around AD 887. He was the son of a Norwegian jarl. Without doubt, it was under his impetus that, from around the start of 10th century, the Scandinavians became attracted to the idea of settling on a long term basis in the area. In AD 911, Rollo started negotiations with the king, Charles the Simple, in order to formalise the Norman sovereignty which already existed de facto in the lower Seine territories. This resulted, in the same year, in the Treaty of Saint-Clair-sur-Epte, in which the Frankish king gave up to the Vikings a territory corresponding roughly to the eventual French départements of Seine-Maritime and Eure.
In return, Rollo accepted Christianity and was baptised, and also undertook to prevent other Vikings from entering the River Seine.
This treaty was the foundation of the Norman state, with Rouen as a pivot, and Rolf as “Jarl of Rouen” (Rudhuborg Jarl). The term ‘Duke’ was not to emerge until around AD 1000. The prerogatives of the Jarl were those of a Frankish count, having most of the attributes of the royal power, what remained of the royal patrimony and control of the Church included.
Simultaneously, other Viking forces settled further west, particularly in Cotentin (mainly Hiberno-Norse from Ireland, and Orkney Vikings) and Bessin (mainly Danes from the English Danelaw). In AD924 and AD 933, the Jarl of Rouen obtained from the Frankish king the transfer of these two territories, plus the one of Avranchin, further south, in order to be able to control these rebellious and very active ‘western’ Vikings. These territorial acquisitions were not completed easily, as these populations had for long resisted any central authority. The Breton occupation period in Cotentin and Avranches (AD 867-927) has left very few traces.
At the conclusion of this expansion, Normandy covered approximately its present-day area. Only two buffer territories remained, which were to be acquired subsequently : the land of Talou, on the Flemish border (in AD 996) and Passais (around Domfront), on the meridianal border (c. AD 1050). The Norman borders have not varied since, representing outstanding stability in a Middle Ages world were borders were complex and constantly fluctuating.