In 1120 Notre Dame of Paris received a fragment of the True Cross from a former canon, Ansel, who had joined the First Crusade and become cantor of the Holy Sepulchre. The history of the relic was sought out, documented, inserted into the liturgy and in time revised according to political and cultural developments. This article reconstructs the story of the relic's arrival in Paris, and the way in which, through the successive narratives attached to it, it slowly gained and grew in meaning, giving definition to the Church of Paris and ultimately to the kingdom of France.