During the 4th and 5th centuries, the Roman practice of coopting Germanic tribes to provide military and defense services along the route from Boulogne to Cologne created a Germanic-Romance linguistic border in the region that persisted until the 8th century. Saxon colonization into the region from the 5th to the 8th centuries likely extended the linguistic border somewhat south so that by the 9th century most inhabitants north of Lille spoke a dialect of Middle Dutch, while the inhabitants to the south spoke a variety of Romance dialects. This linguistic border is still evident today in the place names of the region. After the 9th century, the linguistic border began to shift north and east, a process accelerated by modern government policies that recognize French as the country's official language. There are currently 20,000 speakers of a subset of the Dutch dialect West Flemish in the arrondissement of Dunkirk and this particular subset is in danger of extinction within decades.
Situated in the north of the country along the western half of the Belgian frontier, the department is unusually long and narrow. Its principal city is Lille, which with nearby Roubaix, Tourcoing and Villeneuve d'Ascq constitutes the center of a cluster of industrial and former mining towns totalling slightly over a million inhabitants. Other important cities are Valenciennes, Douai, and Dunkirk. The principal rivers are the following: