The art of building has always been a sacred work. The builders of cathedrals of the Middle Ages developed an esoteric mystique as in the days of ancient Egypt. They moved from town to town, keeping pace with the work offered to them. Pierre Perrat thus worked on the cathedral of Metz, but also on those of Toul and Verdun. The architect had lodges for his tools to prepare his work. The brotherhood of builders was governed by rules which had to respect quality and moral standards. They also preserved the secrets of the trade.