book 1, chapter 3.

BRITTANY: A. D. 992-1237. The First Dukes.

"After the death of Solomon ... all these districts or territories merged in the three dominations of Nantes, Rennes, and Cornouaille. Amongst the Celts concord was impossible. In early times Nomenoe, the Ruler of Cornouaille, had assumed, by Papal authority, the royal style, but the Counts of Rennes acquired the pre-eminence over the other chieftains. Regality vanished. Geoffrey, son of Conan [A. D. 992-1008] ... must be distinguished as the first Duke of Brittany. He constituted himself Duke simply by taking the title. This assumption may possibly have been sanctioned by the successor of Saint Peter; and, by degrees, his rank in the civil hierarchy became ultimately recognized. ... The Counts of Brittany, and the Dukes in like manner, in later times, rendered homage 'en parage' to Normandy in the first instance, and that same homage was afterwards demanded by the crown of France. But the Capetian monarchs refused to acknowledge the 'Duke,' until the time of Peter Mauclerc, son of Robert, Count of Dreux, Earl of Richmond [A. D. 1213-1237]."

_Sir F. Palgrave, History of Normandy and England.,