Historical territory in present-day Belgium, France, and the Netherlands
Source (Wikipedia): County of Flanders

The County of Flanders was one of the most powerful political entities in the medieval Low Countries, located on the North Sea coast of what is now Belgium. Unlike its neighbours, such as the counties of Brabant and Hainaut, it was within the territory of the Kingdom of France. The counts of Flanders held the most northerly part of the kingdom, and were among the original twelve peers of France. For centuries, the economic activity of the Flemish cities, such as Ghent, Bruges and Ypres, made Flanders one of the most affluent regions in Europe, and also gave them strong international connections to trading partners.