The story
During the short-lived Napoleonic Empire, most of the area of the modern province was part of the Department of Escaut, named after the River Scheldt. Following the defeat of Napoleon, the entity was renamed after its geographical location in the eastern part of the historic County of Flanders (now in the western portion of the current Flemish Region).
The provincial flag has a black lion with red tongue and claws, on a background of horizontal white and green stripes. This is a recent adaptation; formerly, East Flanders used the Flemish flag, a black lion on a yellow background, as in the current coat of arms. The old flag is still publicly used, e.g. for road signs.
From Wikipedia’s article on East Flanders, shared under CC BY-SA 4.0.5
