From the Wikipedia page 
Yvelines (French pronunciation: [ivlin]) is a French department in the region of Île-de-France.
Yvelines was created from the western part of the former department of Seine-et-Oise on 1 January 1968 in accordance with a law passed on 10 January 1964 and a décret d'application (a decree specifying how a law should be enforced) from 26 February 1965. It inherited Seine-et-Oise's official number of 78.
It gained the communes of Châteaufort and Toussus-le-Noble from the adjacent department of Essonne in 1969.