This beautiful large blue damselfly is described as a banded demoiselle by Wikipedia, or banded agrion in our old butterfly book. By damselfly standards this is large, around 2 inches long. This one is a brilliant iridescent blue, they can be blue-green. The most distinctive feature is the large dark patch on the wing.
Seen commonly in Europe and Asia. This one paused conveniently by the River Vézère in the Dordogne region of France, late summer 2018.