This is a very old unused black and white/sepia postcard of Cite de Carcassonne - Eglise Saint-Nazaire. Le Chevet. I have no idea of the exact age of the card, but it looks like it is from the very early 1900s.  The card is in very good condition – no creases or obvious marks (other than a couple of very minor ones), with only very slight scuffing to some corners & edges.