(left) The banner used by the Duchy of Brittany; (right) the arms of the Duchy until the ducal title (which continued to be used by French nobility after Brittany's annexation into France) was abolished in the French Revolution.
History
The area known today as Brittany was first occupied by Celtic tribes, then Gallic ones, before the territory was absorbed into the Roman Empire (the famous French comic series, Astérix le Gaulois is set in a village located in Brittany). When the Roman Empire fell, emigrant Briton tribes (part of the reason for the place's etymology) arrived, setting up various little kingdoms that frequently warred with each other, until Charlemagne's Frankish Empire conquered the region in 799 AD. The fall of the Empire after Charlemagne's death, however saw the region resist Frankish attempts to incorporate it, defeating the Franks in 851 AD and securing a period of Breton independence as the united Kingdom of Brittany, marked by waves of Viking raids in the area that lasted until 937 AD, when a descendant of a Breton king, called Alan drove the Vikings out and founded the new Duchy of Brittany.
After expelling the Vikings, in 942 AD Alan offered to become a tributary of France; but the Duchy's early history is mostly marked by rivalry with another duchy on the English Channel, neighbouring Normandy, with whom numerous wars were fought in the 11th century, including also with the Kingdom of England after it's 1066 conquest by the Duke of Normandy, William the Conqueror. These dynastic disputes would result in a 1160 dynastic union with Normandy's successors, the House of Plantagenet, rulers of the Angevin Empire; arranged by England's King Henry II. The Empire would fall apart under his (famously incompetent) son King John, whereupon the Duchy of Brittany went back to being a French vassal in 1202, for the next few centuries.
(above) Breton map of Brittany and it's historical regions; today part of this land, around the city of Nantes, is not included in the administrative division of Bretagne.
The 13th century saw numerous wars of succession in the Duchy, in which France's Capetian kings attempted to annex the Duchy several times; throughout the Hundred Years' War, both England and France attempted to woo the Duchy, as they did the numerous other feudal entities in France (such as Burgundy and Provence). When one of the most powerful of those, the Duchy of Burgundy abandoned their English allies after the Congress of Arras in 1435, Brittany saw the writing on the wall and did likewise, and in 1453 the English suffered their final defeat of the war at the Battle of Castellan, extinguishing all English hopes of restoring the Angevin Empire's continental possessions.
(above) Map of the Duchy of Brittany in 1477, shortly before Anne de Bretagne's marriage to Charles VIII of France.
Afterwards, French influence continued to seep into the Duchy, and in the late 15th century several Franco-Breton wars were fought, in which Brittany was forced to accept humiliating terms effectively making it a French puppet; in 1491 the Duchess of Brittany, Anne de Bretagne, married Charles VIII of France, leading to a dynastic union with France. After his death in 1498, Anne married his cousin and successor, Louis XII of France, who allowed her to reassert Breton independence; but after her death in 1514, the Duchy of Brittany became a possession of the next Queen (consort) of France, Claude, and in 1532 the Estates of Brittany (a parliament of sorts) voted to unite the crowns of France and Brittany. So her son, upon succeeding to the throne of France in 1547, became King Henri II, ruler of both France and Brittany in his own right, ending the Duchy's independence and absorbing it into France.
Coinage
The Brittany issuer in Numista contains coinage issued under both the Kingdom and Duchy of Brittany; both these iterations of the Breton state issued their coinage to the standard set by Charlemagne back in 781 AD, with 1 Livre being a pound of silver, subdivided into units of 20 sous/sols, each of which was worth 12 deniers; all at par with the Livre used by neighbouring France until 1795; by the time of Brittany's annexation into France in 1547, that had become the circulating coinage for a few decades already. Many of the same denominations seen in French livre coinage are also used in Breton coinage, with the same names; below is a list of the common coinage denominations.
Obole = 1/2 Denier
Denier = 1 Denier (duh!)
Double = 2 Deniers (sometimes called a Double Denier)
Blanc = 10 Deniers, or 5/6 of a Sous (a demi, or half blanc denomination also exists, worth 5 Deniers)
Gros = 20 Deniers, or 1 and 2/3rds of a Sous (Gros is the French equivalent of the German groschen)
In 1498 a famous, rare gold coin (pictured below) called a "Cadière" worth 3 Livres was issued by order of the Duchess of Brittany, Anne herself; ostensibly as a move to assert Breton independence (since authorisation to strike coinage was reserved to sovereign rulers) after her second marriage to a French monarch. Those were struck to the same dimensions as the Écu d'Or of France, and would be the final Breton coinage issue.
(above) The obverse and reverse of the "Cadière d'Anne de Bretagne"; this specimen pictured is the one owned by the Musée de Bretagne, which was stolen a few months ago in April 2018 along with a reliquary containing the embalmed heart of Anne, but recovered safely afterwards.
https://en.numista.com/catalogue/bretagne-1.html