From the Wikipedia page 
Yonne (French pronunciation: [jɔn]) is a French department named after the river Yonne. It is one of the eight constituent departments of Bourgogne-Franche-Comté and is located in the northwest of the region, bordering Île-de-France. It was created in 1790 during the French Revolution. Its prefecture (capital) is Auxerre and its postcode number is 89.