Excerpts from Books and Wikipedia
"The Duchy of Normandy, which began in 911 as a fiefdom, was established by the treaty of Saint-Clair-sur-Epte between King Charles III of West Francia and the famed Viking ruler Rollo, and was situated in the former Frankish kingdom of Neustria. The treaty offered Rollo and his men the French lands between the river Epte and the Atlantic coast in exchange for their protection against further Viking incursions." [Wikipedia] "The first of Rollo's descendants to use the title Duke of Normandy was Duke Richard II, grandfather of Duke William (c.1028-87), who later became William the Conqueror, the first Norman king of England." [Furtado: 1001 Days] "Charles the Simple offered Rollo a homeland of his own on the western coast and promised to make him ruler of it. In exchange, Rollo would have to agree to accept Christian baptism, to be loyal to the king of the Western Franks, and to fight against any other Viking invaders who might trouble Charles's kingdom. Rollo agreed to the deal." [Bauer: Medieval World, p. 489-90]