The five black stripes symbolise the ancient bishoprics of Upper (eastern, Breton-speaking) Brittany: Dol, Nantes, Rennes, Saint-Brieuc and Saint-Malo. The four white stripes represent the bishoprics of Lower (western, Gallo-speaking) Brittany: Cornouaille, Léon, Trégor and Vannetais. The top left section of the flag is ermine on a white background.
The ermines are the symbol of the Dukes of Brittany. The order of the ermine was created by Duke Jean IV
Still trying to spot the ermines on the flag? When ermine was used to decorate shields, the white skins were sewn together side by side, and the black-tipped tail placed in the middle of each where it was fixed by three stitches in a cross shape. This became the stylised heraldic symbol seen now on the gwenn ha du and elsewhere.