In certain regions of Brittany, when the foundation stone is laid, a coin minted in the current year is placed in a hollow in the stone.
This tradition, which dates back to the Middle Ages, is intended to instill longevity and divine protection into the building. The coin, a modern French currency such as a euro or a 2 euro coin, also serves as a chronological marker, capable of revealing the age of the building should it ever be demolished or renovated.