Nantes was the principal capital of the Duchy of Brittany until the sixteenth century .
Rennes had been the seat of the Duchy 's supreme court of justice between 1560 and 1789 .
Although Nantes was the principal capital of the Duchy of Brittany until the sixteenth century , Rennes had been the seat of the Duchy 's supreme court of justice between 1560 and 1789 .